I'm no audio/eletric technician but I know a bit about amplifiers, maybe you'll give a penny for what I think it is:
If the amplifier makes the loud pop even when no cable is connected, you bet it's the amplifier. I would return it, if your warranty expired I would send it to a technician for evaluation, maybe it's no big deal. Sometimes the switch has some poor contact, he'll swap to another one and problem solved.
Inadequate voltage or voltage oscilations in the energy coming out of your wall outlet might be causing this too. Try using a surge protector or try plugging it to a UPS no break, the surge protector regulates the voltage an eletric device receives by blocking or grouding the voltage so that it is safe for the equipment. Same goes for the UPS, but it will condition and equalize the voltage more precisely. The pop you hear might be some extra voltage, this might damage the device pretty bad.