I had the same problem with an Ayre V3 amp. I first built a DC blocker and that did not help so i removed the transformers that were humming and took them to a motor shop. They were dipped in that special winding varnish and when i reinstalled them they were dead quiet. What happens is that one or more loops of wire loosen up and then the magnetics cause the loose wire to vibrate. By locking them all back together it solves the issue.
