Can moving wire location at circuit breaker reduce hum?

     A lot of pro (instrument) amps have phase-reverse switches, to deal with hum issues, on stage.

     Those suggestions on the outlet wiring tester, to me, would be a good/basic place to start.

     Hots of all outlets in the room, on the same leg, in the panel/breaker box?

                                                    My 2 cents.

Ground loop you have..Tom

Thinking back over 25 years ago I had a client with top of the line Melos gear. Had the same hum issue and voltage on a line it turned out to be a coax cable hooked to a piece of gear connected to the preamp. Tom

Thanks, everyone, for your input.  I’ll pick up the tester on Amazon.

I have shut off the circuit that goes into my music room and went around testing the outlets all around it to see which ones are dead.  I’m guessing the receptacles would be in the same general area so I didn’t go to every receptacle in my house.

I’ve then disconnected everything except the amp and it’ll still hum.  I’ve had numerous electronic components hum in this particular room.  In the room just next door, dead quiet.  
 

Hair dryers, kitchen appliances, dimmers,... can cause this even by using dedicated lines for your system. One easy way to check is heat produced from the transformer when idling or with no signal (check preamp also). Try to relocate and eliminate possible suspects as advised above or get a dc blocker, it works.