Quote:
Scott thought that it had something more to do with the power lines or surge of more power coming thru the electrical lines at different times of the year rather than something being wrong with the init.


I would agree with Scott...
I have the same situation, at times, with my 7500. Its been like that since day one (2005-6); When I first noticed it, I had tried everything to track it down. I also had talked to Support about it. Never came up with a definitive answer except that, yes, it could be power transmission to my house.
The hum appears to happen at very busy times of the day (suppertime, early morning)...it may last 10 minutes, or 10 seconds...it is a very weird, BUT almost predictable pattern.
Like you indicated, it is not loud, i.e., I have to be near the amp to hear it. I hate to admit it, but I kind of just live/or base my HT/music episodes with the hum.....but as indicated, it is a very sporadic occurrence.
Not because of this incident, but soon a friend of mine is going to do a more thorough re-wire of my HT room (dedicated circuits, more receptacles, etc); like you, I plan on doing an upgrade of my preamp.
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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