Power amp humming - help please

Gong, is the hum coming from the speakers or the amp directly? If it is coming from the amp directly then I agree, it is most likely the transformer. If it is coming from the speaker then you probably have a ground loop. Try putting a ground "cheater" on the amp's power cord and see what happens.

The volume doesn't change when I crank the volume up on pre, if that means anything...

mijostyn, excuse my extreme ignorance on Hifi, but how do I put a ground cheater on the amp's power cord?

The hum is coming from the speakers and not the amp, mijo...

Right!!!