Is there, by chance, a relay on that board, to prevent the power-on
> pop from getting through to the amp?
>
> If so, you might want to investigate that part. It is very common for
> these to go bad and only let volume scratch through.
It's late at night.. :) I see from a second readover that you have already investigated that. Maybe if I look a third time during the day I can figure the rest of it out.