I don't know if it can help but one thing I never do is run the wires from the psu to the amp boards in series, I always use separate power wiring for each amp board and join them at the power supply. You seem to know what you're doing so it's just my humble 2 cents. I can't remember why I do this but I think I had some problems with that kind of wiring in the past with two Beta22 boards and I have been avoiding it ever since. Maybe it's just builder's superstition
I think that if you changed the stock gain you also need to change the compensation capacitors.