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.