Buzz coming from power amp - not through speakers

The PA5800 is not an amp I would put too much work into. When I owned one,there was a distinct drop in overall volume level anytime I switched from two-channel playback to multi-channel. I replaced it with an Onkyo AVR, which was a surprisingly better performer.

I picked it up on ebay for $135, so I don't have much invested in it. I wouldn't be able to hear it over music, but it was audible while watching TV due to the pauses and breaks in the audio. I'll probably just live with it for a while and upgrade to something a bit nicer a few months down the line.

If you are using it with a TV, then you very possibly have a ground loop problem related to your cable or satellite box.
I use the L & R audio outputs from my satellite box to feed my receiver and not the outputs from the TV. I just mute the TV speakers.

Yep, I that's what I was doing. RCA outs from the directv box to a Marantz AV7005 and into the H/K, with the TV speakers set to zero. I would be running everything HDMI through the Marantz but I don't have cables long enough yet. I'm going to be rearranging the whole room over the next couple weeks, so it's possible that a new outlet and/or different location in the room could clear it up.

I may be interferece getting on your power lines . The transformer in that amp is apparently more susceptible to it than other amps. Check for any flourescent lights or dimmers being used that use the same circuit. It could be getting worse each time your refrigerator or heating unit kicks on too. Try to isolate the amp circuit from the digital and tv circuits.
I had a very high end integrated that had a similar "periodic" noise and the solution ultimately was teh manufacturer redesigned the transformer. In this case, it is not an option so the "preventive" measures are to attempt to minimize the noise to a tolerable level.

A power conditioner may help but my experience is it doesn't. I had some success with a PS Audio humbuster but not always. The humbuster is good at stopping DC noise but not much else and usually it is not "just DC noise" you are getting.

The gentleman who recommended "tack" I believe was referring to "Blu-tac" which is an expensive version of mortite (also available at places like HD or Lowes, much cheaper and just as effective). Basically you would use it to damp the transformer to tame the vibrations. With a toroidal transformer it is usually the internal windings that are vibrating so the noise will not stop but the damping may at least reduce the "volume".
HTH