If all else has failed, you might try this (no guarantees):
With preamp off and unplugged from line voltage, remove top and loosen transformer bolt enough to allow transformer rotation. If transformer wiring is tight due to multiple wire ties that make wiring look "neat", cut wire ties. Plug preamp back in, turn on, and get it to where you hear the hiss. Rotate transformer to find the null spot where the hiss is the quietest...there should be 2 null spots, 180 degrees of transformer rotation apart from each other (either that or the transformer rotation just doesn't do anything at all). Turn preamp off, disconnect from line voltage, tighten transformer bolt, "pretty" up wiring if you want, put top on, re-install in system.
Good luck
Dan