60Hz line noise was another problem in the weeds of the noise floor.

Putting a ground line around the 6.8k resistors gave a questionable improvement. The biggest improvement came from

moving the power & ground lines away from the pots & the twisted pairs.  This kind of noise reduction isn't going to be consistent, as it's transported.  Digital pots & flood filled grounds are necessary .

Transient pops are still a problem in the aux channel.  Disabling phantom power & running the preamp on batteries didn't fix it.  Still debating upgrading the preamp to 100x.