[Uncle Erik]

You can try an isolation transformer - those suppress ground loops and some electrical noise. An "audiophile" model is not necessary. You can find any number meant for electronics hobbyists (to keep from frying yourself) or hospital gear that works fine. Nebraska Surplus has a wide variety of isolation transformers at good prices.

If you are dealing with a RFI problem, that's a little trickier. Shielded cables can lock the noise somewhat, but distance is usually the best cure. If you can relocate your equipment away from each other, that can solve the problem.

 

[userlander]

I plugged the amp into an extension cord from across the room this morning, and no noise. The extension cord only takes two-pronged plugs, so it had an adapter on it. So then I had the idea to plug the amp into the powerstrip again *with* the adapter, and no noise! I verified by trying it again without the adapter, and the noise is there without it. So there must be some weird ground loop problem.

I think I'm going to get adapters for all my gear on these outlets. I wonder if there is interference affecting my computer or network or other stuff. I've never noticed any noise from my speaker amp, but it can't hurt to be sure.

thanks everyone for helping me figure that out.