Sometimes the hum is caused by "DC Offset". The AC power quality is off a little, and the transformer has to work harder to get the right power to the amplifier or other component. It will vibrate more when it has to work harder. A power regenerator/ voltage corrector would probably fix this kind of hum. (PS Audio, Equi=tech, etc.) Sometimes Stehno's solutions will work as well, as well as a dedicated line, if the power problem is in your house and not the local utility.
Amplifier Transformer Hum
If you narrow the hum down to the amplifier's transformer, try to determine which circuit the amplifier is running off of. Then check the circuit breakers in the service panel.
Chances are that your amps' circuit is on the opposing phase of some or all of your other audio circuits and therefore generating some AC noise thats affecting the transformer.
Have an electrician move all of your audio circuit breakers in the service panel to the same phase of 115 volts.
Just for grins I moved my amp's circuit over to the opposing phase where my other dedicated audio circuits are to see what sonic improvements I may find. No sonic improvements that I could tell but it cured my transformer hum and the transformer has been almost dead silent every since.
Chances are that your amps' circuit is on the opposing phase of some or all of your other audio circuits and therefore generating some AC noise thats affecting the transformer.
Have an electrician move all of your audio circuit breakers in the service panel to the same phase of 115 volts.
Just for grins I moved my amp's circuit over to the opposing phase where my other dedicated audio circuits are to see what sonic improvements I may find. No sonic improvements that I could tell but it cured my transformer hum and the transformer has been almost dead silent every since.