Knownalien - Let me weigh in this discussion. I have had some experience with ground loops. My hobby is electronics and over the years I have developed a passion for amplifiers. the amplifiers I have connected to my 990 are all hand built and provide some 2000 RMS watts of power all without a detectable hum.

The existance of a ground loop in your system invariably shows up in the input stage and therefore receives the full amplification of your amplifier and a very audible noise from your speakers.

Ground loops are caused by small variations in the resistance of the wires that return the signal to ground. Like the design of any high power amplifier, your system there should only have a single point where everything is grounded. When you connect the 990 to the 7500, the cables set the appropriate ground level for the 7500. Therefore, all cables should be of the same quality and they should all be about the same length. If the cable company has grounded your system, you probably may want to reconsider any other grounds you might have implemented.

Since the 2200 are mono blocks you would only be connecting one cable to them without any possibility of a gound loop (there is only one ground).

Outside of that I would switch some of the cables from the 990 to the 7500 to see if I could get the hum to move arround. You may detect a faulty cable. Failing all of these things, and an unwilling cable company, I would start suspecting the 7500, a loose wire or something.

I once had a small ground loob in my dual subwoofer amplifier. I got all of my test equipment out and started to trace it down, only to find out that it was being caused by a "Y" connector that I had used to split the signal. The one foot "Y" connector had just enough resistance differences to set up the ground loop. After I directly connected both amps to the 990, the hum disappeared.

You may want to google "ground loops" there are some excellent articles on this subject out there.