090128-1818 EST
You need to determine the origin.
Is it inductive pickup from inductive fields?
Is it capacitively coupled?
Is it from ground path current?
How do you determine the source. If I have an amplifier contained in one shielded box, supplied from a wall outlet, a speaker connected to the amplifier, no wires connected to earth or an EGC except the one grouind wire in the power cord, and all gain controls set to 0, then is there hum? If so it is an internal amplifier problem. You might need to short any microphone inputs for this test.
If in the above test there is no hum, then attach things and determine what causes hum. When you find the item that causes the hum, then determine why.
Bad filter capacitors in the amplifier should probably produce 120 Hz hum.
.