Supertom...is the hum coming directly from the amplifier itself? Does the unit continue to hum if you remove all input and output cables from the amp? (Please do this with the power OFF!)

My friend's computer makes noises very similar to your amp. In her case it's a bad power supply fan. If there's no fan in your amp...and the hum is indeed coming from inside the amplifier itself...the problem may be a loose lamination or winding on the power transformer.

Ground loop hum is something you would hear coming from the speakers.

BobZoom