How to get rid of transformer hum?

Usually physical damping of the transformer(s) helps a lot. Make certain the they are tightened down securely to the chassis. You might consider using some type of spacer if they will not tighten down properly. I have seen people place (wedge) hard rubber doorstops between power transfomers (not power supplies) that were spaced close together with good results. Just be carefull as these things can hold a lot of current even after they have been powered down for long periods of time. Anyway, I would try this out first before going the technical route.

PS: As far as using soft footers directly under the units, I disagree with Steve's comment. I have an external power supply on a Musical Fidelity amp (that sometimes hums). Using a soft footer in this manner just isolates the vibration to the unit which deteriorates the sound in this case. I have found it much more effective to draw some of the vibration from the unit to a Maple platform with the use of cones. However the platform is isolated with soft footers (Vibrapods) so that other vibration is not introduced to the platform. I will be simplifying this setup with the use Neuance shelving, but still recommed the use of cones/platform/soft footers as an inexpensive fix on amps and power supplies.

If it is a DC bias then a isolation transformer may help. If the isolation transformer wants to hum because of to high voltage then it may have a higher voltage tap you can try. You could also move the isolation transformer to the breaker panel. I have a 5kVA unit that I intend on wiring tothe breaker panel and then put my dedicated outlet on it.

Guys thanks for the feedback, I do have two diodes invertered, and prior to these amps I had Cary SLAM 100s which didn't rattle at all. I did speak with Nelson Pass and he is the one that gave me the filter idea. I can't tighten the transformers anymore so I'm looking for a way to clean up the sine wave. I may have to resort to a conditioner by PS Audio or Exact Power. Any other ideas.

Cleaning up the sine wave certainly won't hurt anything, but oddly enough it may not help. Magnetostriction is related to volt/seconds. The transformer stores up energy during each half cycle. If you begin to push the storage capacity to its limit it begins to saturate and hum. Since you are stuck with 60 hz only the volts are available to change. It is the area under the curve and not the shape of the curve that counts for volt/seconds.

It would be interesting to know what your line voltage is. Use an RMS reading voltmeter if you can, rat shack sells them along with some nice speaker cable (opps getting off the subject :-) if you don't have one of those a regular volt meter will work OK. If the line voltage is high then the following option might help.

There are ferroresonant transformers you can buy for probably 3 to 5 hundred bucks that clean up the waveform and stabilize the voltage. They also provide tremendous surge protection. The problems are that they are (at least the ones I have seen) very noisy. This is because they deliberately run them in saturation. So, you would have to locate the transformer somewhere away from your listening area. They also waste alot of power. Something like a third of their rating. So it is best to disconnect them when not using your system.

A step down transformer would be cheaper. It can be an autoformer, since you are not looking for isolation. An autoformer has no secondary. This makes them smaller and therefore cheaper.

Try four or five more of those diode pairs. If they don't help you are not out much. Try the cheapest stuff first.