Are you sure it is a ground loop? Perhaps the amps just hum.
Go to Radio shack and get 2 RCA connectors. Short the center connection to the ground connection with a clip lead or paperclip or something. Or cut a cheap interconnect, strip it back a little and twist both wires together. Plug them into the input of the amp and turn it on with nothing else plugged in except the speaker. If it still hums it is the amp.
What is "fairly audible?" Can you hear hear it at the listening position. How close do you have to get to the speaker to hear it?
Go to Radio shack and get 2 RCA connectors. Short the center connection to the ground connection with a clip lead or paperclip or something. Or cut a cheap interconnect, strip it back a little and twist both wires together. Plug them into the input of the amp and turn it on with nothing else plugged in except the speaker. If it still hums it is the amp.
What is "fairly audible?" Can you hear hear it at the listening position. How close do you have to get to the speaker to hear it?