When I hooked up my 7700 I had the that terible hum. I went crazy looking for it and I have two Monster cable surge protectors. The 3600 and 3500. I also have comcast cable.

AvFan had a good idea about disconnecting everything and then reconnect to get the head of the problem.

My situation called for a dedicated 20 amp circuit for the 7700. Once I did that my system was quite as a mouse. I really had no other options since I had eight outlets on two 15 amp circuits and was running a lot of equipment. Not a hell of a lot of amps but enough to cause that HUM.
If you are still having problems as I am writting this try running a new electrical line.
It is not hard to do as long as you have easy access to your walls