I've got one of these in the shop right now.  There is a high frequency oscillation that is being fed to the power amp (causing it to heat up with no input and results in the buzz you hear. Pulled the PA (LM3876T) and the oscillation is present on the input to the chip, so back in the signal path from that.

What's irritating is the board mounted SMD chips don't have valid part numbers on them so it's going to be a reverse engineering job if the company won't cough up schematics and parts specs.

I'll have to ask the owner if he wants to go ahead with an expensive repair on an $350 amp or just leave it as an extension speaker (which he's already done)