Some people recommend to isolate all gear and lift the shields on one end of all balanced lines. This eliminates all possible secondary ground paths, and should technically reduce or eliminate hums. I'd get a painful shock if I touched both amps. If the chassis were grounded together, you would have blown a fuse before possibly electricuting someone. I personally keep all equipment tied together for grounds.
All power comes off of a single circuit.
If you use multiple circuits, the different lengths of wire can reduce the voltage a little bit more than another circuit, which will cause hums. Another circuit might also use the other phase, and the AC will be completely out of phase, and cause problems too. I occasionally have to use an audio isolation transformer to kill a hum, by I feel that it's a safer method. These all tend to put crap back in the circuit. Large transformers tend to cause a bit of hum.