Don't get crazy...I think we all become anal about our systems; I know that I've have been super-sensitive about ground loop issues because it is a major bugaboo with high-end systems....and I originally had problems with it when I first set up my 990/7500 combo years ago.
As I've said several times in different posts on the Saloon, I'm pretty sure my amp "hum" issues are related to incoming power from the electric company "Grid". Lately, I have had no issues at all, for like 2 weeks. When I do get them, it is at a certain time of day (supper-time), and lasts intermittently for about an hour. That's it...everything is dead silent after that, for days on end. Although I did have an issue recently, and it turned out to be an old lamp, with a weird kind of bulb that was in another room (not even on the same circuit!).
In my eyes, these Outlaw amps are bullet-proof...I really believe your amp hum issue is a grounding/electricity thing; are you sure the Comcast line coming in is properly grounded?