[paulsherwin]

It would be good to work out why they're failing. A quiescent amp shouldn't be dissipating much power in the output transistors, so it may be the switchon surge that's taking them out. You could try it with a lamp limiter in place but I imagine you're running out of transistors to try now.

If there are no other faults then I agree the transistors are suspect. I usually try to sub known types I have in stock rather than trying to chase up obscure originals which may be fakes. Transistors are much less variable than is often assumed and subbing usually works well.