[Granny Gremlin]

If all (or any of) those transistors weren't properly insulated wouldn't something a bit more drastic be happening than volume drop and distortion? I mean been there, done that, blew the power supply (in this case most likely one or both of the fuses as the power rail just pours into ground?).

 

[Paul Running]

 

[jb welder]

Does your amp have the case insulators installed on the power transistors?
View attachment 388630

Those are there to protect the user from the voltage on the transistor casings. Mica insulators are still required between the transistors and the chassis.

If all (or any of) those transistors weren't properly insulated wouldn't something a bit more drastic be happening than volume drop and distortion? I mean been there, done that, blew the power supply (in this case most likely one or both of the fuses as the power rail just pours into ground?).

Usually this is correct. But you can still have broken or missing mica insulators and the heatsink compound (and in this case chassis paint) 'mostly' insulates. Then the paint or heatsink grease can act like a resistor and you get some leakage through to chassis and they might just run hot. Not necessarily the case here but it's easy to check that they are in place and not damaged/cracked.