You could also try isolating it. Try several layers of good Scotch 33 or 88 electrical tape between the metal to metal mounts. Tighten the screws back down enough to make it secure, but not super tight. Try this method if the above doesn't work. The cabinet might be amplifying the noise.
New Transformer possible?
I have a Beard 500 tube preamp,I believe it was made in early 80s,I really love the way it sounds,fantastic highs,mids are spot on bass is tight and tunefull.The problem is noise from the transformer,its just from the preamp,But it is loud enough to hear from my chair. I hear no noise of any kind from my speakers.I would like to know if a new transformer would take care of the noise/hum.I could always get a new preamp, but to find one that sounds as good as this one would be tough,or very expensive.Any help advise would be appreciated. Thanks MAXWELL