You may have a ground loop problem, but I'm a bit confused by a "hum" coming from a tweeter. Anyway, try a cheater plug to eliminate the ground on the amp or the receiver and see if that takes care of the problem. The cheater is not the best way to run an amp of course, but it will let you know if the problem is caused by a ground loop.
Hum from new amp?
I just bought a used Belles 150A 3 channel amp to drive my B&W N805's HTM2 center. I noticed a very slight hum coming from the tweeter.It cannot be heard from more than 6" away from the tweeter. It is not affected by the volumn control. Right now I have the Belles amp sitting directly on top of the Marantz SR7000 which is being used as a pre-pro and to drive the rears. I am using double runs of VDH CS-122 for speaker wire and Cardas Quadlink IC's. The Amp came with what seemed to be a heavy power cord. Both the amp and receiver are plugged into a 6 plug power outlet with surge protection. Please make some suggestions to fix this problem. BTW the sound quality is greatly improved by the Belles amp. The soundfield is much wider and voice are more in front of the speakers. Separation is much better and things are bigger and wider sounding. Voices have more depth and emotion.