Identify this hum, please

Added a new (used) mono pair of amps which didn't hum in the system I heard it in. Previous amps haven't hummed in my system at all. It hums equally from both channels, gets louder as you turn up the volume. Tried lifting grounds with cheater plug, no change. Tried moving it to a different circuit using an extension cord and it hums and buzzes there. What could it be?