The last time I repaired a power amplifier a few years back, it required new output transistors as one channel's devices had blown. When I removed them, there was merely some powdery remains of what used to be heatsink compound left under them, so I lifted the other channel's devices whilst I was in there and replaced the compound under them, as it was just as bad.
As a result, whenever giving an amplifier a general service, I always have a poke round in the general direction of these devices to try and ascertain what sort of condition it is in. More often than not, it's in a bad way and benefits from renewing.
As an aside I've just had a brainwave -audiophile heatsink compound!
