Just to confirm - nothing plugged into the amp and it is humming (no cord/patch cables)? Something like
this can give you a quick diagnosis of your ground/polarity. I have several monitors, 1 desktop, 1 laptop, printer, wireless access point, a ceiling fan, PC speaker, wireless chargers and my amps running in my home office/music room and they are dead quiet - quiet enough I can DI record from my pedalboards or amps(solid state or tubes) with almost no noise floor. You may have faulty wiring or a faulty device. If you have a circuit in the house with no other devices, plug the amp in it to see if you are getting the same hum. Kind of concerning your PC speakers are humming too - mine are dead quiet.