Finally got PCBs ordered. Probably two weeks before they arrive. I'll do a full write up on this project later. The short version is eight on board transformers and BA amplifier circuit with separate panel control board connected via 8 position ribbon cable. Multiple boards can be cascaded together. Minimal off board wiring. Adjustable trimmers for setting input drive akin how tape machines could be calibrated so that certain channels compliment different instruments' frequency response. These also be fixed resistors or wired off board for more panel control options.
Requires separate summing amplifier and PSU running 24V-48V/0V, or bipolar up to +/-24V. I've ordered a couple more than the minimum just in case their is still interest when they arrive.
Thanks!
Paul