The SVT II has a protection circuit built in that has failed on one I worked on before. It would randomly cut out and turn off. I ended up bypassing the circuitry and it has been running great ever since.
Even if a couple tubes "went bad", they would have blown the internal resistors etc and the amp would still work.
Might try another "repair" shop because that one doesn't sound very promising.
I think you may be right, I spoke with another well respected local tech, and while he said he could possible see the 500 charge if you replaced all the tubes, with the bench fee, from what I described he said it didn't even sound like a tube problem, im going to take it to him next week, he will also thouroughly go over the entire amp to make sure nothing else needs fixed or replaced, the original place I have taken it may be right, and may be qualified, but I don't know that, so im not taking a chance

