If you are getting a mechanical buzz from the transformer that would be caused by physical vibrations. Why don't you try placing a brick on top of the transformer and see if the extra weight damps the vibrations. Vibrating transformers can be the result of some DC on you incoming power line. PS Audio at one time made a device called the Humbuster that filtered that out and from what I remember reading about it, did a good job of reducing transformer buzz. Unfortunately they have been out of production for a while now and are not easy to find on the used market.
Also, try just plugging the amp into an outlet in a different part of the house (or even a different outlet in the same room, if it is on a different breaker) to at least see if the problem is coming right out of the wall. No need to connect anything up, just listen to see if the transformer buzzed when plugged into a different breaker.
But that aside, I don't see how a mechanical transformer buzz would make its way to the electrical signal going to your speakers to produce a buzz there. Do you have access to a scope so you can take a look at what the PS output looks like? You make actually have two separate problems.