It sounds as though the receptacle from which the amplifier is run with the extension cord might be missing its ground connection.
The 48 and 66V readings are likely caused by capacitive coupling (if you're using a high-impedance meter) and/or filter components connected from line to ground in the amplifier.
The fact that deliberately lifting the ground (with the adapter) removes the hum seems to suggest either a ground loop of some kind or that the building ground is carrying a lot of noise.
The hum is still there with no inputs connected to the amplifier and the input gain controls all turned to minimum, right?
Is there a possibility that you have a ground on the speaker wiring somewhere?
[This message has been edited by pauluk (edited 11-05-2003).]
