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?
dhcod
Its not the amps if the volume control makes a difference. Does it hum if the volume is all the way down?
Try the cheater plug on the preamp and also try a different set of interconnects from the preamp to the amps.
Are your preamp and cd on the same electrical circuits?
Is there a cable connection (from tv) that is routed through your system?
Tried different interconnects?