What's hummin'? The amp, or is the hum coming out of the speakers when the amp is on?
If it's the amp itself, check your grounds and try the old process of elimination by removing other stuff on the circuit. Can't really add anything to what's been said already.
If it's through the speakers, (again) definitely try the cable -- especially if you had the same problem with two amps -- as it don't sound like the amp(s) are to blame at all. Ground loop hums off of a coaxial cable line are very common. If you have a cable line running anywhere into the system (or anything even remotely connected to the amp) I'd bet you dollars to doughnuts that's your culprit. If that's indeed the problem, Radio Shak makes an coaxial RFI blocker that's mean to fix just that sort of problem. Bought one, didn't work worth a damn. Useless. Mondial also makes a "magic block" that's supposed to do the trick. It'll cost you $100, though (which, all things considered, is far more cost effective than a $20 Rat Shack paper weight that doesn't work -- or even weigh enough to hold down paper). Finally, as mentioned by Senoir Rudd, there is also the Jensen VRD-1FF -- which worked like a charm for me (and made me very happy). And if it doesn't, they'll give you your $60 back. Best of luck.