It's strange you got such a high voltage across R12. I don't remember something like that on either of the two channels in mine.
Have you measured the pots? They could be damaged and it would be well worth the time to do a diode test on the mosfets, just to see of they have shorted.
One more thing, I don't recommend prolonged use of the F5 with a low ratio Stax transformer at loud levels. It got, very, very toasty...