There is a little gadget called a cheater plug, available in hardware
stores. When a power cord is plugged into the cheater plug, and the plug is
inserted into the socket so that the little ground tab on it touches
nothing, the result is frequently the elimination of stubborn hum loops.

People can talk about "black background", and related stuff I'll never
understand. But a hum loop spoils my day, and this $0.35 contraption solves
a lot of them.

Sure, people will tell you it's dangerous. If you use a cheater plug, don't
use your amp as a place to park your ass. Don't grab it while searching for
a water pipe to hold onto. Turn the system off before you make mods. In any
event, you can plug the cheater plug into a ground fault circuit
interrupter, which will provide protection similar to what the troublesome
ground connection provided, without the hum.