I always look at the simplest solutions first when dealing with a hum situation. This is a odd question but is there a cable tv box plugged in anywhere nearby. If so try unplugging it. I had a ton of hum in my stereo system when my cable box was plugged in and after I pulled the plug and any connections that went to it it went away. Also do you have a short in a cable maybe try another one.
This gremlin could be a lot of things, try removing a piece of gear at a time to try and narrow it down.
The cable box, or any other electrical appliance on the same circuit, even in another room, can cause noise. Also check the cables, they can also cause noise. Have you checked the ground settings on all things?