is there any way of intercepting the signal path as it passes from the pre-amp to the power stage?
Sounds like a good place to start,try cleaning all the send and return FX loop jacks, especially the shorting contacts which will require removing the amp from the cab(make sure its unplugged).Also worth cleaning the master vol pot.Sounds unlikely to be the power transistors as they usually go dead short when they blow and then take out the power supply fuses.
If none of these work then best to take it to a proper techie.
Good luck
Cheers Just