Are you a psychic?

I knew you were going to say that

Also, if you go into the kitchen and look in the cutlery drawer (near the sink), I sense you will find spoons, possibly some of them bent.

Anyway. The possible effects of an ungrounded wall socket can include induced noise on audio equipment, mainly because a lot of consumer equipment has high leakage current from the PSU.

IE, in most TV sets, the power supply outputs float at approximately half the ac line voltage. This is mostly down to cost reasons, as a fully isolated PSU is more expensive. The end result is that the ground points on the TV, such as the outer of RCA connectors, are also at half the mains supply although at very low current levels, typically microamps. Enough to feel especially in europe, where half the mains voltage is still 110V, but not normally dangerous.

If somewhere between the offending device and the amp you have a faulty earth, sometimes enough of the leakage current can get into the works and manifest itself as a signal at whatever the mains frequency is, 50 or 60 Hz. It might even be the amp itself that is not properly earthed. VCRs are often culprits in this way as well.

It might be worth trying unplugging everything, then connecting each audio source in turn into the amp directly, with both amp and source powered from the same wall socket. See if any combination hums. If not, add more stuff until the problem comes back.

If you are running anything from an extension cable, that might also have a faulty ground. Also, it is possible that during the move an RCA cable got damaged and has a broken ground connection, which wouldn't help. Check all the cables with a multimeter if possible.

Finally, try plugging everything into a different set of socket in another room.

Oh yes, one final though I just had: If the live and neutral wires were transposed on any socket, that could cause all sorts of strange results under the right circumstances, as could a neutral and earth transposition. I've seen both, and neither is safe never mind useful.

Patrick