So it is probably one source connected to your system causing the hum. I know it is a pain, but I suggest disconnecting everything and methodically plugging things back in beginning with the Denon and then the 7700 until the hum appears. The idea is to isolate the piece of equipment that is sensitive to hum or where the hum originates.

One additional thought, and this depends upon your skill level with electrical (and daylight!), you might want to check the grounding in your dedicated circuit. If you don't like or are familiar with electrical I'd hire an electrician to check your electric panel, outlets, etc to make sure everything is ship shape. A good electrician should be able to diagnose and repair a grounding issue that may be the cause of the hum in your speakers.

And lastly, its generally not the amp that is the problem, its the way our electrical components are hooked up that causes the hum.