Evidently I've some slider caps to hunt out yet.
Living in a box didn't do the mixer a whole lot of good, as evidenced by the bent PCB for channel seven. The wear & tear is limited to aesthetics, and thankfully that's invisible once everything is buttoned up.
I either forgot to count up the caps in the headphone amplifier circuit, or they've been misplaced. Since this is a less than critical point I ignored it (along with a couple in the LED bar graph driver circuit) so I could reassemble the mixer and move on to the next thing.
At some point in the future I'll wrap up another year + old project!