Thanks to all for the helpful suggestions and thoughts! One of the things that warms my heart about all of this is how delicately everyone is handling the possibility that this could be going on inside my ears, or even the rest of the way inside my head. I don't think that it is, but I appreciate how much trouble everyone is taking, not to just call me nuts and leave it at that. Pics will follow when I'm back home, though that will take a few hours.

The rig has been, at various times, out in the wide open of the room, and stored in a horizontal object from which the back has been completely removed and the doors are left open when the rig is running. No two pieces are stacked. Leaving the room and coming back hasn't resulted in an alleviation of the problem, and low listening levels only attenuate the issue to the extent that they attenuate the sound, proportionately.

One thing that seems to be true that I'm not altogether sure is true, is that when the problem is happening there seems to be a different sonic signature to the low-level hum that I can always hear emanating from the speakers. When the problem is happening, the hum seems to have a fuzzy, almost rapid-clicking quality that doesn't seem to be there when the problem isn't happening. The amplifier makes a consistent hum, both down the speaker wires and through its own front panel, and the preamp makes a very loud bang if accidentally disconnected from AC-mains while the amp is on, and the cd-player transformer runs extremely, nay disturbingly hot to the touch, even after short listening sessions -- all of which could be indicative of gear that's having trouble dealing with its power.

Of all the reasons that I don't think this is an "all-in-my-head" problem, the facts that non-audiophiles have heard the difference, and that the earlier trouble with the old Parasound stuff was not repeatable on any of *three* test benches, suggest to me that what's going on is something specific not so much to my head as to my house.

Only one power conditioner has ever been used, but it's been both used and *not* used, with no noticeable difference. One thing that hasn't been tried yet is an isolation transformer. The problem has not been tested using headphones.

As for ground, it's been suggested by another forum member elsewhere that I might not have a sufficient earth ground for my entire house wiring system, or massive amounts of DC offset, and that these root causes could at least explain why so many different problems seem to be cropping up in so many different configurations of gear: whatever the problem from moment to moment, the system is unable to dissipate it down the safety ground the way it otherwise would, and hence is dealing with it by acting-up. This other person suggested that the hum from the amp, the banging sound from the preamp, and the hot transformer for the cd-player, could all be different ways of coping with an inability to dissipate fault energy down the safety ground.

The recurrence of the problem at the friend's house seems explicable for one of two reasons or possibly both: (a) If the root-cause had already compromised one or more pieces of equipment, (b) if the root-cause is something generic to the entire local grid, such as massive DC-offset.

Incidentally, my friend does have a stubborn problem with his rig, too, and it dissipates every time he disconnects and reconnects his own stack. His LP's get reedy and unpleasant to listen to, until he does exactly the same thing.