Those pesky ground loops will drive you crazy! I assume you didn't have the hum until introducing the 7700 so maybe you have a cable issue. Do you get the hum from every speaker and is it at the same volume? If the hum is only in a one or two speakers you might have a cable issue due a bum cable. However, if the hum is the same in every speaker it is more of a system issue. By that I mean a single source is probably the problem. It doesn't sound like it, but you might have a noisy electrical environment near your equipment too. Do you have any other electrical devices located nearby that could induce a signal in the RCA cables between the Denon and the 7700? This is kind of a longshot since your cables use RG6 coax.

I'd start by unplugging everything and then plug in the Denon and then the 7700. I'd try this with and without the Monster 2600. If the hum returns at that point you narrowed the issue to those two components and a call to Outlaw may be necessary. If not, keep adding components until the hum appears.

One more thing: does the hum occur day and evening? There was a Gunslinger that had the hum in the evening but not during the day. It turned out it was a bad switch or fixture used to light his backyard at night on a separate circuit that induced a hum in his gear.

Keep the Saloon updated on your progress.