In my home theater I have 4 Altec 9440A amplifiers mounted in a metal relay rack along with my surround sound processor and DVD/CD player. This is a new arrangement and now I am starting to fight ground loops. If I connect the left and right channels out of my processor to one of the amplifiers, there is no hum. If I add another amplifier from my processor for the center channel, there is quite a bit of hum. Any amplifier by itself is quite, it is just when I try to add the second amplifier that the hum starts. When I have time, I am going to take the 4 amplifiers out of the metal rack along with the processor and see if the hum is still there when I add multiple amplifiers. The 9440A is the most musical amplifier that I have every heard. I like it so well that if I cannot solve the ground loop problem I will have a home theater with 2 channels.
Note that I am trying all of this with unbalanced cables.