How to play Rocksmith 2014 Remastered on Linux using the USB guitar adapter?

Rocksmith 2014 Edition - Remastered on Linux (specifically NixOS 19.09) does not detect the Real Tone Cable. I've tried some things from protondb:

1. I've set Steam to use the latest Proton release, 4.11-8, for this game.
2. I've run sudo ln -s ~/.local/share/Steam/userdata / although that seems like a weird idea.
3. sed -i.orig 's/ExclusiveMode=1/ExclusiveMode=0/;s/Win32UltraLowLatencyMode=1/Win32UltraLowLatencyMode=0/' /path/to/Steam library/steamapps/common/Rocksmith2014/Rocksmith.ini
4. Set "Rocksmith Guitar Adapter" to off in the pavucontrol "Configuration" tab.
5. Set the game launch options to PROTON_NO_D3D11=1 %command%.
6. I also need to set hardware.pulseaudio.extraConfig = "default-fragments = 5\ndefault-fragment-size-msec = 2\n";, but that looks like latency settings rather than being related to the detection of the cable.

7. WINEPREFIX=~/.steam/steam/steamapps/compatdata/221680/pfx winetricks sound=alsa fails with the following error message:

   wine cmd.exe /c echo '%ProgramFiles%' returned empty string, error message "wine: '…/.steam/steam/steamapps/compatdata/221680/pfx' is a 64-bit installation, it cannot be used with a 32-bit wineserver."

   I don't know why this happens or how to fix it. I've tried to add WINEARCH=win64 to the command, but that doesn't seem to change anything. I also tried making the same change using protontricks, but for some reason that doesn't detect this game at all.