Ground Cheater plug or....

Yes all devices you have are grounded, some are chassis grounds (2 prong devices) and some externally(3 prong). The problem is that these grounds are just different enough to create a small current flow when they are on the same circuit and that is what you hear as 60 Hz hum. Rewiring all to be 3 prong will most likely fix, but that is pretty extreme. Not saying this is your issue, just that it was mine.

So here's what fixed it. I switched pre-amps, and suddenly the amp was quiet, but when I turned on the pre-amp the hum came back. It had mysteriously jumped between components! To finally get it to settle down, I stuck a cheater plug between the pre-amp and the power supply - the PS is still grounded at the wall - and that fixed it. I don't hear any sonic degradation from the cheater. Cheaters seem to be controversial, but at this point, and since the PS is still grounded, I'm not arguing. But hell's bells. it took a year to figure out.

Did you ever try shorting the amp's inputs, as I had suggested? Doing that would probably have led to the conclusion (which now appears to have been the case) that the hum that was heard while nothing was connected to the amp's inputs did not have the same cause as the hum that was heard while the preamp was connected. Knowing that would probably have led to a much quicker resolution. See my post dated 3-30-12.

Regarding the safety aspects of using a cheater, IMO it's your call. While the risk is very small, assuming that the equipment is in good condition, it can't be said that the risk is zero. And if that very small risk were to materialize the consequences could be very serious.

Regards,
-- Al

Well again, the power supply - a PS Audio something-or-other - is still grounded. The cheater is between the preamp and the power supply, and the power supply still has a protection circuit, so I think the risk is pretty low. The amp, which is the big power draw, is still grounded.

I might check on the polarity thing though. It couldn't hurt.