I have a 990/7500 setup...I have no hum at all in my system (its been up for about 3 weeks now)....dead silence. I use a DIRECTV H10 hi-def receiver; also have a rooftop OTA antenna conected to this receiver. My house is old (1925) with variations of modern wire, romex, "knob and tube"; all my gear is connected to a couple of surge protectors (reasonabally good ones) going into a couple of outlets, tied to 15A breaker. All receptacles are three prong, though. It's weird why this problem is affecting some users and not others.
_________________________
Anthem MRX-500/ Outlaw 7500, Paradigm Studio 100's Fronts, Paradigm CC-590 Center, Phase Technology Dipole rears , Martin Logan Dynamo sub, Oppo BDP-103, DIRECTV H22-200 DVR HD receiver, Samsung PN60E8000 3D Plasma 60"
2nd System
B&K ST 202 Amp, Outlaw 990, Rotel RCD 1072CD, Yaqin CD 1 Tube Buffer, Apogee Centaur Minors, Phase Technology Model 10 Sub, Rotel RP-855 turntable, Panasonic DMP-BD30, DIRECTV H21-700 DVR HD receiver, Free Agent Home Theatre+, Vizio 40" LED