Perkri,
my system had one amp plugged into an outlet with the pre-amp and the other amp across the room into another outlet. The amp that was across the room was making the 60 Hz cycle sound, pretty loud at pauses between songs. I disconnected from the pre-amp and no buzz. switched channels...you know go through the gambit of isolating the source. The answer was that the power chord is grounded, three prong, so is the other amp and the pre-amp. I bought two converters that eliminate the ground - three prong to two prong and all the buzz went away. I left the pre-amp grounded. Apparently the grounding was traveling through the source cables back to the pre-amp and causing the cycling noise.
Hopefully this helps, but if not Tubes 4 Hifi - Roy Mottram roymottram@yahoo.com can help you nail it down. If it's an amp issue, Joe Servini (think that's his last name) k3tye@earthlink.net is awesome. Had to send my amps to him because I couldn't figure out why they were sounding so lean. Joe went through it and found that the factory coloring on the transformer wires were incorrect and didn't match the instructions from the kit. Amazing it didn't melt down. It's now up and running for a few years and sounds better and better as the caps smooth out. have about 500 hours on it and now issues at all.
good luck, I've heard stories about the ground loop being caused by a coffee maker plugged into a wall outlet in a different room.