I had a BAT tube preamp with hum so bad, I quit using it and went back to SS. I gave it to a tech to check. When I returned home he called and had put it on his test equipment and told me it was better than spec. I told him it was mostly a mechanical hum from one of the toroidal transformers, and he replied," I know about that". He loosened the central transformer bolt, rotated it on axis for minimum noise and gently retightened the bolt and fixed the noise while I was on line with him. I had, like you tried everything. You may be able to do the same thing, with a little luck......or it may be some other problem. This should have no effect on your warranty, but if you have a warranty, why are you worrying about this?