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.
Keep power amps and mixer power supplies away from audio equipment. Where possible, keep signal wires and AC wires seperated, and allow them to cross only at 90 degree angles. Also, Your nearfield monitors should be placed about 6'-10' apart, directly centered on your position at the console. The center lines of the speakers should be angled together, converging on a spot about a foot behind your head. Some people like to spread their nearfields even more, to emphasize the stereo image.