How to get rid of transformer hum?

Lets see what we got so far. The mechanical hum couldn't been a design problem, for that would have surfaced and licked at the maker's QC dept.
I agree with Stevemj 5-14-01 post part about loose laminations or windings acting up. Varnish which is used to fill gaps in trans. or motor windings do deteriorate with time. Tommy, it might be worth your while to call a motor/transformer coilwinder workshop to check if all you need is a bake and re-varnish to seal off your misery! This problem is very common thing to electrical maintenance people. Get opinions of a couple of coil winders & their price to the job.
I've not had this same problem with my audio so far, but I have solved similar transfr hums (in my work) by tightening mounting screws or a re-bake/varnish at the winder's. I cannot guaranty this will work for you but a coat of insulating varnish does no harm, and I dont think is expensive. But try not start your conversation with the winder saying the usual hi-end audio stuff, you know what i mean :)

Tommy, have you tried to power these amps at another local? Away from your home? I saw that the mfg stated that its a power problem. I may not be an EE, but I have over 20 years in the field and 9 times out of 10 its not the equipment or the power but another source of noise on the line saturating the transformer causing it to hum. I know we all hate the power company and sometimes they are at fault. I'm just curious as to whether the problem follows the amps when they are powered somewhere else.

Here is the diode thing with a filter.
http://www.hifi-notes.com/ahacoffsetkiller1-en.htm

Mrderrick

The amps were at another home with the same power company. They didn't hum there. I listened to the my parasound amps in my surround system and they hum too, they are behind a wall and the transformers are about 10% of the size of the Pass. I've had the power company out and I saw the sine wave on there scope it was chopped off at the top. Just not enough for them to work on fixing it.

While it may not be a feasible solution for some, try moving. The transformer in my DAC hummed audibly (hearable from ~2 feet away) and it was almost on its way to the factory for repair. There was also a seemingly unrelated hum through the speakers that floating the preamp's ground removed, though the overall sound was better with the ground enabled.

Well, I moved last week. Bigger and much more modern place. Especially the wiring. Previous place was a unit in a '50's era fourplex; new one is a "condo" (though NO shared walls) that's about ten years old.

When initially set up in the condo the DAC still hummed and the pre-problem was actually more pronounced. A couple of days later the DAC hum is completely gone and the speaker hum caused by the pre- is only audible from a scant few inches away.

Can't explain it, but what a relief!