You have actually maybe found the problem. Your amp does not like the power it is supplied. You are talking about a mechanical hum right? If not, and it hums out your speakers, then try a cheater plug on your amp to remove the ground.
For mechanical hum, you could try one of those things that fix the power (not a conditioner). Products like the Powervar, Blue Circle Music Ring, Equitech, Furman, PS Audio Power Plant, etc.. Waring that sometimes these devices will solve your gear hum, but then hum themselves.