If anyone has a spare CM621UB they're willing to part with for cheap or free, I'd love to have one to do some bench testing, and to try making a line-out mod.
JohnNevets: Nice work on the combo moves! The service manual does list "3+6+SOUND" for through mode, but it doesn't show 3+4 and 3+5 for sedan/wagon modes. 3+6 worked once for me, but I never saw it again because I didn't know about the other two. It all makes sense now.
flyboy1100: All of the audio specs in the service manual have a note that says "auto loud off" and "eq through mode", so I'm pretty sure through mode means flat EQ.
As for loudness, I can see why you might want to leave it on. It used to just add insult to injury, but with the EQ disabled, it's much more reasonable. I can change it on the fly with one hand now, but I think I'll mostly be leaving it off; I find the tonal balance often isn't great across volume levels and recordings, so I end up fussing with the tone controls. With loudness off, I can just keep the volume within a range where the tonal balance sounds right for a given recording and leave it at that.
One the way to work this morning, I caught myself just enjoying the music and not thinking about the system. I didn't think this was going to be possible with the stock setup.