When you say new boards, do you mean that you have replaced the amplifier boards?
You could try swapping the boards - if the problem stays on the same channel it is the wiring or OP transistor, if the problem moves it is on the board.
I'd guess that a single OP transistor failure would result in severly distorted sound, but that *is* just guessing.
If you still have the original amplifier boards you could try swapping them back.
