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.
There is the possibility that your amps' circuit is on the opposing leg 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 leg of 115 volts.
Just for grins I moved my amp's circuit over to the opposing leg 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.
There is the possibility that your amps' circuit is on the opposing leg 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 leg of 115 volts.
Just for grins I moved my amp's circuit over to the opposing leg 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.