I would also suggest a shielded power cord for the phono preamp. Check its p/c to see if it is marked shielded (this is in addition to it being a 3 wire cord). Shunyata's Venom or Diamondback are fully shielded (braided shield) and reasonably priced.
I'm not fullly aware of your situation but this was an easy fix for a friends hum problem with his phono preamp.
