One hum problem solved, another remains

I had recently relocated my equipment to another room, where I can now integrate my 2 channel gear into a multichannel rig. In my old room I never noticed any sort of ground hum problem, but I was faced with two when I turned on my stuff in this new setup.

The first one was easy to solve. It was apparent only on my phono input and increased in volume with an increase in preamp volume. Pretty loud, too. It turned out that I accidentally connected the ground lead from my phono cable to my phono preamp. In my particular setup this produces a real buzz. So that one was now taken care of.

The second is noticable on all inputs, does not increase with volume, and really can just barely be heard with my ear about 1 foot away from either speaker. But I don't remember it in the old room. In this new setup the amp is plugged into a different outlet from the rest of my gear. I tried flipping the polarity of the plug on the amp with no success. Every other piece of equipment is on a three prong plug and is plugged into a Wiremold L10320. I don't know if the two different outlets are on the same breaker.

Any help would be appreciated.