diff --git a/corrscope/gui/__init__.py b/corrscope/gui/__init__.py
index bc64689..447b038 100644
--- a/corrscope/gui/__init__.py
+++ b/corrscope/gui/__init__.py
@@ -32,6 +32,8 @@ from corrscope.gui.model_bind import (
     rsetattr,
     Symbol,
     SymbolText,
+    BoundComboBox,
+    _call_all,
 )
 from corrscope.gui.util import color2hex, Locked, find_ranges, TracebackDialog
 from corrscope.gui.view_mainwindow import MainWindow as Ui_MainWindow
@@ -289,7 +291,7 @@ class MainWindow(qw.QMainWindow, Ui_MainWindow):
         if name:
             master_audio = "master_audio"
             self.model[master_audio] = name
-            self.model.update_widget[master_audio]()
+            self.model.update_all_bound(master_audio)
 
     def on_separate_render_dir_toggled(self, checked: bool):
         self.pref.separate_render_dir = checked
@@ -580,7 +582,7 @@ def nrow_ncol_property(altered: str, unaltered: str) -> property:
         if val > 0:
             setattr(self.cfg.layout, altered, val)
             setattr(self.cfg.layout, unaltered, None)
-            self.update_widget["layout__" + unaltered]()
+            self.update_all_bound("layout__" + unaltered)
         elif val == 0:
             setattr(self.cfg.layout, altered, None)
         else:
@@ -637,8 +639,12 @@ class ConfigModel(PresentationModel):
     master_audio = path_fix_property("master_audio")
 
     # Stereo flattening
-    combo_symbol_text["trigger_stereo"] = list(flatten_no_stereo.items())
-    combo_symbol_text["render_stereo"] = list(flatten_modes.items())
+    combo_symbol_text["trigger_stereo"] = list(flatten_no_stereo.items()) + [
+        (BoundComboBox.Custom, "Custom")
+    ]
+    combo_symbol_text["render_stereo"] = list(flatten_modes.items()) + [
+        (BoundComboBox.Custom, "Custom")
+    ]
 
     # Trigger
     @property
@@ -826,6 +832,7 @@ class ChannelModel(qc.QAbstractTableModel):
         Column("wav_path", path_strip_quotes, "", "WAV Path"),
         Column("amplification", float, None, "Amplification\n(override)"),
         Column("line_color", str, None, "Line Color"),
+        Column("render_stereo", str, None, "Render Stereo\nDownmix"),
         Column("trigger_width", int, None, "Trigger Width ×"),
         Column("render_width", int, None, "Render Width ×"),
         Column("trigger__buffer_strength", float, None),
@@ -865,7 +872,7 @@ class ChannelModel(qc.QAbstractTableModel):
     # data
     TRIGGER = "trigger__"
 
-    def data(self, index: QModelIndex, role=Qt.DisplayRole) -> qc.QVariant:
+    def data(self, index: QModelIndex, role=Qt.DisplayRole) -> Any:
         col = index.column()
         row = index.row()
 
diff --git a/corrscope/gui/model_bind.py b/corrscope/gui/model_bind.py
index 1c926fa..71769d3 100644
--- a/corrscope/gui/model_bind.py
+++ b/corrscope/gui/model_bind.py
@@ -1,5 +1,6 @@
 import functools
 import operator
+from collections import defaultdict
 from typing import *
 
 import attr
@@ -10,8 +11,8 @@ from PyQt5.QtWidgets import QWidget
 
 from corrscope.config import CorrError, DumpableAttrs, get_units
 from corrscope.gui.util import color2hex
-from corrscope.utils.trigger_util import lerp
 from corrscope.util import obj_name, perr
+from corrscope.utils.trigger_util import lerp
 
 if TYPE_CHECKING:
     from corrscope.gui import MainWindow
@@ -37,6 +38,12 @@ Symbol = Hashable
 
 SymbolText = Tuple[Symbol, str]
 
+
+def _call_all(updaters: List[WidgetUpdater]):
+    for updater in updaters:
+        updater()
+
+
 # Data binding presentation-model
 class PresentationModel(qc.QObject):
     """ Key-value MVP presentation-model.
@@ -54,7 +61,7 @@ class PresentationModel(qc.QObject):
     def __init__(self, cfg: DumpableAttrs):
         super().__init__()
         self.cfg = cfg
-        self.update_widget: Dict[str, WidgetUpdater] = {}
+        self.update_widget: Dict[str, List[WidgetUpdater]] = defaultdict(list)
 
     def __getitem__(self, item: str) -> Any:
         try:
@@ -76,8 +83,11 @@ class PresentationModel(qc.QObject):
 
     def set_cfg(self, cfg: DumpableAttrs):
         self.cfg = cfg
-        for updater in self.update_widget.values():
-            updater()
+        for updater_list in self.update_widget.values():
+            _call_all(updater_list)
+
+    def update_all_bound(self, key: str):
+        _call_all(self.update_widget[key])
 
 
 SKIP_BINDING = "skip"
@@ -137,7 +147,7 @@ class BoundWidget(QWidget):
 
             if connect_to_model:
                 # Allow widget to be updated by other events.
-                model.update_widget[path] = self.cfg2gui
+                model.update_widget[path].append(self.cfg2gui)
 
             # Allow pmodel to be changed by widget.
             self.gui_changed.connect(self.set_model)
@@ -147,6 +157,8 @@ class BoundWidget(QWidget):
             perr(path)
             raise
 
+    # TODO unbind_widget(), model.update_widget[path].remove(self.cfg2gui)?
+
     def calc_error_palette(self) -> QPalette:
         """ Palette with red background, used for widgets with invalid input. """
         error_palette = QPalette(self.palette())
@@ -179,7 +191,9 @@ class BoundWidget(QWidget):
     gui_changed: ClassVar[Signal]
 
     def set_model(self, value):
-        pass
+        for updater in self.pmodel.update_widget[self.path]:
+            if updater != self.cfg2gui:
+                updater()
 
 
 def blend_colors(
@@ -212,6 +226,7 @@ def model_setter(value_type: type) -> Callable[..., None]:
         except CorrError:
             self.setPalette(self.error_palette)
         else:
+            BoundWidget.set_model(self, value)
             self.setPalette(self.default_palette)
 
     return set_model
@@ -285,6 +300,9 @@ class BoundComboBox(qw.QComboBox, BoundWidget):
     combo_symbol_text: Sequence[SymbolText]
     symbol2idx: Dict[Symbol, int]
 
+    Custom = object()
+    custom_if_unmatched: bool
+
     # noinspection PyAttributeOutsideInit
     def bind_widget(self, model: PresentationModel, path: str, *args, **kwargs) -> None:
         # Effectively enum values.
@@ -292,9 +310,13 @@ class BoundComboBox(qw.QComboBox, BoundWidget):
 
         # symbol2idx[enum] = combo-box index
         self.symbol2idx = {}
+        self.custom_if_unmatched = False
 
         for i, (symbol, text) in enumerate(self.combo_symbol_text):
             self.symbol2idx[symbol] = i
+            if symbol is self.Custom:
+                self.custom_if_unmatched = True
+
             # Pretty-printed text
             self.addItem(text)
 
@@ -302,7 +324,12 @@ class BoundComboBox(qw.QComboBox, BoundWidget):
 
     # combobox.index = pmodel.attr
     def set_gui(self, model_value: Any) -> None:
-        combo_index = self.symbol2idx[self._symbol_from_value(model_value)]
+        symbol = self._symbol_from_value(model_value)
+        if self.custom_if_unmatched and symbol not in self.symbol2idx:
+            combo_index = self.symbol2idx[self.Custom]
+        else:
+            combo_index = self.symbol2idx[self._symbol_from_value(model_value)]
+
         self.setCurrentIndex(combo_index)
 
     @staticmethod
@@ -316,7 +343,9 @@ class BoundComboBox(qw.QComboBox, BoundWidget):
     def set_model(self, combo_index: int):
         assert isinstance(combo_index, int)
         combo_symbol, _ = self.combo_symbol_text[combo_index]
-        self.pmodel[self.path] = self._value_from_symbol(combo_symbol)
+        if combo_symbol is not self.Custom:
+            self.pmodel[self.path] = self._value_from_symbol(combo_symbol)
+            BoundWidget.set_model(self, None)
 
     @staticmethod
     def _value_from_symbol(symbol: Symbol):
@@ -445,6 +474,7 @@ class _ColorText(BoundLineEdit):
         self.setPalette(self.default_palette)
         self.hex_color.emit(value or "")  # calls button.set_color()
         self.pmodel[self.path] = value
+        BoundWidget.set_model(self, value)
 
     def sizeHint(self) -> qc.QSize:
         """Reduce the width taken up by #rrggbb color text boxes."""
diff --git a/corrscope/gui/view_mainwindow.py b/corrscope/gui/view_mainwindow.py
index 9372a69..c9dfe58 100644
--- a/corrscope/gui/view_mainwindow.py
+++ b/corrscope/gui/view_mainwindow.py
@@ -174,9 +174,15 @@ class MainWindow(QWidget):
                 with add_row(s, "", BoundComboBox) as self.trigger_stereo:
                     pass
 
+                with add_row(s, tr("Downmix"), BoundLineEdit, name="trigger_stereo"):
+                    pass
+
                 with add_row(s, "", BoundComboBox) as self.render_stereo:
                     pass
 
+                with add_row(s, tr("Downmix"), BoundLineEdit, name="render_stereo"):
+                    pass
+
             with append_widget(s, QGroupBox) as self.dockStereo_2:
                 set_layout(s, QFormLayout)
 
diff --git a/corrscope/wave.py b/corrscope/wave.py
index c891746..b33d096 100644
--- a/corrscope/wave.py
+++ b/corrscope/wave.py
@@ -1,55 +1,114 @@
 import copy
 import enum
-import warnings
-from enum import auto
 from typing import Union, List
 
 import numpy as np
 
 import corrscope.utils.scipy.wavfile as wavfile
-from corrscope.config import CorrError, CorrWarning, TypedEnumDump
+from corrscope.config import CorrError, TypedEnumDump
 
 FLOAT = np.single
 
+# Depends on FLOAT
+from corrscope.utils.windows import rightpad
+
 
 @enum.unique
-class Flatten(TypedEnumDump):
+class Flatten(str, TypedEnumDump):
     """ How to flatten a stereo signal. (Channels beyond first 2 are ignored.)
 
     Flatten(0) == Flatten.Stereo == Flatten['Stereo']
     """
 
     # Keep both channels.
-    Stereo = 0
+    Stereo = "stereo"
 
     # Mono
-    Mono = auto()  # NOT publicly exposed
+    Mono = "1"  # NOT publicly exposed
 
     # Take sum or difference.
-    SumAvg = auto()
-    DiffAvg = auto()
+    SumAvg = "1 1"
+    DiffAvg = "1, -1"
+
+    def __str__(self):
+        return self.value
+
+    # Both our app and GUI treat:
+    # - Flatten.SumAvg -> "sum of all channels"
+    # - "1 1" -> "assert nchan == 2, left + right".
+    # - "1 0" -> "assert nchan == 2, left".
+    def __eq__(self, other):
+        return self is other
+
+    def __hash__(self):
+        return hash(self.value)
 
     modes: List["Flatten"]
 
 
+assert "1" == str(Flatten.Mono)
+assert not "1" == Flatten.Mono
+assert not Flatten.Mono == "1"
+
+FlattenOrStr = Union[Flatten, str]
+
+
+def calc_flatten_matrix(flatten: FlattenOrStr, stereo_nchan: int) -> np.ndarray:
+    """ Raises CorrError on invalid input.
+
+    If flatten is Flatten.Stereo, returns shape=(nchan,nchan) identity matrix.
+    - (N,nchan) @ (nchan,nchan) = (N,nchan).
+
+    Otherwise, returns shape=(nchan) flattening matrix.
+    - (N,nchan) @ (nchan) = (N)
+
+    https://docs.scipy.org/doc/numpy/reference/generated/numpy.matmul.html#numpy.matmul
+    '''
+    If the second argument is 1-D,
+    it is promoted to a matrix by appending a 1 to its dimensions.
+    After matrix multiplication the appended 1 is removed."
+    '''
+    """
+
+    if flatten is Flatten.Stereo:
+        # 2D identity (results in 2-dim data)
+        flatten_matrix = np.eye(stereo_nchan, dtype=FLOAT)
+
+    # 1D (results in 1-dim data)
+    elif flatten is Flatten.SumAvg:
+        flatten_matrix = np.ones(stereo_nchan, dtype=FLOAT) / stereo_nchan
+
+    elif flatten is Flatten.DiffAvg:
+        flatten_matrix = calc_flatten_matrix(str(flatten), stereo_nchan)
+        flatten_matrix = rightpad(flatten_matrix, stereo_nchan, 0)
+
+    else:
+        words = flatten.replace(",", " ").split()
+        try:
+            flatten_matrix = np.array([FLOAT(word) for word in words])
+        except ValueError as e:
+            raise CorrError("Invalid stereo flattening matrix") from e
+
+        flatten_abs_sum = np.sum(np.abs(flatten_matrix))
+        if flatten_abs_sum == 0:
+            raise CorrError("Stereo flattening matrix must have nonzero elements")
+
+        flatten_matrix /= flatten_abs_sum
+
+    assert flatten_matrix.dtype == FLOAT, flatten_matrix.dtype
+    return flatten_matrix
+
+
 _rejected_modes = {Flatten.Mono}
 Flatten.modes = [f for f in Flatten.__members__.values() if f not in _rejected_modes]
 
 
 class Wave:
-    __slots__ = """
-    wave_path
-    amplification offset
-    smp_s data return_channels _flatten is_mono
-    nsamp dtype
-    center max_val
-    """.split()
-
     smp_s: int
-    data: "np.ndarray"
-    """2-D array of shape (nsamp, nchan)"""
+    data: np.ndarray
 
-    _flatten: Flatten
+    _flatten: FlattenOrStr
+    flatten_matrix: np.ndarray
 
     @property
     def flatten(self) -> Flatten:
@@ -64,13 +123,13 @@ class Wave:
         return self._flatten
 
     @flatten.setter
-    def flatten(self, flatten: Flatten) -> None:
+    def flatten(self, flatten: FlattenOrStr) -> None:
         # Reject invalid modes (including Mono).
-        if flatten not in Flatten.modes:  # type: ignore
+        if flatten in _rejected_modes:
             # Flatten.Mono not in Flatten.modes.
             raise CorrError(
                 f"Wave {self.wave_path} has invalid flatten mode {flatten} "
-                f"not in {Flatten.modes}"
+                f"not a numeric string, nor in {Flatten.modes}"
             )
 
         # If self.is_mono, converts all non-Stereo modes to Mono.
@@ -78,6 +137,8 @@ class Wave:
         if self.is_mono and flatten != Flatten.Stereo:
             self._flatten = Flatten.Mono
 
+        self.flatten_matrix = calc_flatten_matrix(self._flatten, self.stereo_nchan)
+
     def __init__(
         self,
         wave_path: str,
@@ -87,28 +148,26 @@ class Wave:
         self.wave_path = wave_path
         self.amplification = amplification
         self.offset = 0
+
+        # self.data: 2-D array of shape (nsamp, nchan)
         self.smp_s, self.data = wavfile.read(wave_path, mmap=True)
 
         assert self.data.ndim in [1, 2]
         self.is_mono = self.data.ndim == 1
-        self.flatten = flatten
         self.return_channels = False
 
         # Cast self.data to stereo (nsamp, nchan)
         if self.is_mono:
             self.data.shape = (-1, 1)
 
-        self.nsamp, stereo_nchan = self.data.shape
-        if stereo_nchan > 2:
-            warnings.warn(
-                f"File {wave_path} has {stereo_nchan} channels, "
-                f"only first 2 will be used",
-                CorrWarning,
-            )
+        self.nsamp, self.stereo_nchan = self.data.shape
 
-        dtype = self.data.dtype
+        # Depends on self.stereo_nchan
+        self.flatten = flatten
 
         # Calculate scaling factor.
+        dtype = self.data.dtype
+
         def is_type(parent: type) -> bool:
             return np.issubdtype(dtype, parent)
 
@@ -152,16 +211,7 @@ class Wave:
         data: np.ndarray = self.data[index].astype(FLOAT, subok=False, copy=True)
 
         # Flatten stereo to mono.
-        flatten = self._flatten  # Potentially faster than property getter.
-        if flatten == Flatten.Mono:
-            data = data.reshape(-1)  # ndarray.flatten() creates copy, is slow.
-        elif flatten != Flatten.Stereo:
-            # data.strides = (4,), so data == contiguous float32
-            if flatten == Flatten.SumAvg:
-                data = data[..., 0] + data[..., 1]
-            else:
-                data = data[..., 0] - data[..., 1]
-            data /= 2
+        data = data @ self.flatten_matrix
 
         data -= self.center
         data *= self.amplification / self.max_val
diff --git a/tests/test_channel.py b/tests/test_channel.py
index 4fa8cc8..314d758 100644
--- a/tests/test_channel.py
+++ b/tests/test_channel.py
@@ -145,6 +145,7 @@ def test_config_channel_width_stride(
         [Flatten.Stereo, None],
         [Flatten.SumAvg, Flatten.Stereo],
         [Flatten.Stereo, Flatten.SumAvg],
+        [Flatten.Stereo, "1 0"],
     ],
 )
 def test_per_channel_stereo(
diff --git a/tests/test_wave.py b/tests/test_wave.py
index 2680c66..f2a9e80 100644
--- a/tests/test_wave.py
+++ b/tests/test_wave.py
@@ -8,7 +8,7 @@ from delayed_assert import expect, assert_expectations
 
 from corrscope.config import CorrError
 from corrscope.utils.scipy.wavfile import WavFileWarning
-from corrscope.wave import Wave, Flatten
+from corrscope.wave import Wave, Flatten, calc_flatten_matrix
 
 prefix = "tests/wav-formats/"
 wave_paths = [
@@ -43,6 +43,55 @@ def test_wave(wave_path):
 # Stereo tests
 
 
+def arr(*args):
+    return np.array(args)
+
+
+def test_calc_flatten_matrix():
+    nchan = 3
+
+    # Test Stereo
+    np.testing.assert_equal(calc_flatten_matrix(Flatten.Stereo, nchan), np.eye(nchan))
+
+    # Test SumAvg on various channel counts
+    np.testing.assert_equal(calc_flatten_matrix(Flatten.SumAvg, 1), [1])
+    np.testing.assert_equal(calc_flatten_matrix(Flatten.SumAvg, 2), [0.5, 0.5])
+    np.testing.assert_equal(calc_flatten_matrix(Flatten.SumAvg, 4), [0.25] * 4)
+
+    # Test DiffAvg on various channel counts
+    # (Wave will use Mono instead of DiffAvg, on mono audio signals.
+    # But ensure it doesn't crash anyway.)
+    np.testing.assert_equal(calc_flatten_matrix(Flatten.DiffAvg, 1), [0.5])
+    np.testing.assert_equal(calc_flatten_matrix(Flatten.DiffAvg, 2), [0.5, -0.5])
+    np.testing.assert_equal(calc_flatten_matrix(Flatten.DiffAvg, 4), [0.5, -0.5, 0, 0])
+
+    # Test Mono
+    np.testing.assert_equal(calc_flatten_matrix(Flatten.Mono, 1), [1])
+
+    # Test custom strings and delimiters
+    out = arr(1, 2, 1)
+    nchan = 3
+    np.testing.assert_equal(calc_flatten_matrix(",1,2,1,", nchan), out / sum(out))
+    np.testing.assert_equal(calc_flatten_matrix(" 1, 2, 1 ", nchan), out / sum(out))
+    np.testing.assert_equal(calc_flatten_matrix("1 2 1", nchan), out / sum(out))
+
+    # Test negative values
+    nchan = 2
+    np.testing.assert_equal(calc_flatten_matrix("1, -1", nchan), arr(1, -1) / 2)
+    np.testing.assert_equal(calc_flatten_matrix("-1, 1", nchan), arr(-1, 1) / 2)
+    np.testing.assert_equal(calc_flatten_matrix("-1, -1", nchan), arr(-1, -1) / 2)
+
+    # Test invalid inputs
+    with pytest.raises(CorrError):
+        calc_flatten_matrix("", 0)
+
+    with pytest.raises(CorrError):
+        calc_flatten_matrix("1 -1 uwu", 3)
+
+    with pytest.raises(CorrError):
+        calc_flatten_matrix("0 0", 2)
+
+
 def test_stereo_merge():
     """Test indexing Wave by slices *or* ints. Flatten using default SumAvg mode."""
 
@@ -75,7 +124,7 @@ def test_stereo_merge():
     check_bound(wave[:])
 
 
-AllFlattens = Flatten.__members__.values()
+AllFlattens = [*Flatten.__members__.values(), "1 1", "1 0", "1 -1"]
 
 
 @pytest.mark.parametrize("flatten", AllFlattens)
@@ -107,7 +156,7 @@ def test_stereo_flatten_modes(
     assert nchan == len(peaks)
     wave = Wave(path)
 
-    if flatten not in Flatten.modes:
+    if flatten is Flatten.Mono:
         with pytest.raises(CorrError):
             wave.with_flatten(flatten, return_channels)
         return
@@ -135,9 +184,9 @@ def test_stereo_flatten_modes(
             else:
                 pass
         # If SumAvg, check average.
-        else:
-            assert flatten == Flatten.SumAvg
+        elif flatten == Flatten.SumAvg:
             assert_full_scale(data, np.mean(peaks))
+        # Don't test custom string modes for now.
 
 
 def assert_full_scale(data, peak):