However, if you are still getting the buzz from the speakers with no cd player, or anything else such as a cassette deck / tuner etc. connected to the amp, then your problem is NOT a ground loop.
Either you have appallingly bad mains, and the amplifier's power supply cannot filter out the rubbish, or, much more likely, the amplifier is defective in some way.
Surely hum would be at 50hz and would be coming through the woofer? It shouldnt be possible to hear hum through the tweeter. I still think there may be a problem with the amp.