take them to a friends house.
no input needed, just hook to speakers, plug in:
hum? it’s the amps.
if amps, analyze differences if any l/r time before hum starts? time hum changes?
no hum, it ain’t the amps.
go from there with confidence it is or is not the amps.
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btw, have you hooked them up to an easily movable small set of speakers, different 2 way crossover, any difference?