OK, most likely this is a phase issue.
Common when you have 2 components of your system plugged into 2 separate outlet strings that are on 2 separate breakers and each of them breakers is on a separate leg...
Most of the time it is a TV, since they are sometimes far from the av rack, also subwoofers are prone to this...
Simple to figure out, run an extension cord (with a ground) to the tv or component out that is not on the same breaker. if it goes away that is your issue, dont worry you can move the breaker, or hire someone to do it rather, if you never did it before... another way to figure it out faster is to just unplug the hdmi or anything else that connects it...
Now if that is not the issue, try taking away any coax that comes from the street or dish, etc...
If that makes it go away they sell devices to isolate them...
Now a poor ground can also be the issue, you can test this by running a wire from your ground to your system, ground the amps chassis and see if it goes away, or have an electrician check it out for you... If you have old non grounded cables in the house, think about adding a 20a 12g circuit for your system...
I have hunted these things for a while, I never found it being wires too close or touching, it was always either a single piece of equipment that once removed also removed the hum, or once I had the issue with the separate phases of power, I had a bad ground before, etc...
If everything is plugged into the same outlet, UNPLUG everything from everywhere, get an mp3 player as a source plugged directly into your receiver and see if there is a hum there, if it is not, start adding components until it comes back, the last component added is the culprit.
If the hum is there with a basic source, receiver setup, then I would plug it in somewhere else..