How to get rid of transformer hum?

Well, I think the transformers are either mismanufactured or poorly designed. When you design a transformer you make a deal with the devil. If you use lots of turns the transformer will be quiet, but the large number of turns means smaller longer wire which causes higher resistance so the transformer becomes less powerful. Or, you can make the transformer bigger without increasing the power, that costs more money.

If in fact the transformer is humming because it doesn't have enough turns on the primary, then the only thing one can do that I know of is to lower the voltage to the amp. This could be verifided by using a variac to lower the voltage and see if the hum goes away. More diodes will lower the voltage some. Or a stepdown transformer that dropped the voltage from say 120 to 110 votlts. Of course the step down transformer might hum. I presume you have two diodes in parallel connected anode to cathode. Each time you add one of these it will drop the line voltage about .6 volts. It also increases the lines harmonic distortion but that shouldn't be a problem.

If the hum is being transmitted into the chassis you might reduce hum by placing the amps on something soft. If there is a cover that is vibrating you could try removing it.

I realize that you have measured a 5% sine dip. But are you certian that this is the cause of the hum? I have tracked down a lot of hum problems and most are generated from within the home. I will relate my own personal experience. I always had a slight hum in my tv and Tice isolation transformer for my gear. I lived with it until I bought a Classe CA400. The amp hummed very loud! I finally isolated the source to my heat pumps outdoor unit. Something in the prehaet circuit of the compressor. Right now I just turn it off when I listen. The background is dead silent! I'm going to look into it deeper when I get the unit pm done. I would try turning things off in the house first to see if the hum is caused internally. (off means off at the breaker) Good luck!

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.