Thought I'd 'resurrect' this thread as I'm having the hum/buzz from my 7500. Never noticed it before, but now that I've moved my amp I notice it. Starts as a cyclic hum/buzz (softer to louder) then after the amp's on for a couple hours it's at a constant level that you can hear when the volume is muted. Amp is cool as can be, and no noise from the speakers. Is the only 'fix' a ~$300 filter (either PS Audio Humbuster II or the Channel Island XDC-2)?