Weird, but worth a shot is to try grounding your preamp to a piece of digital equipment, like your CD player or DAC. If that doesn't work, try the same, but with the ground from the turntable to the digital piece.
Grounding Hum
I recently got a new TT for Christmas. It's an MMF 2.2 LE. I think there's a grounding problem or something because there is a slight buzzing noise present. I looked through past threads on the topic here, but nothing seems to fit this situation. I've checked, and rechecked the connection between the TT, and the preamp. I've even used a Fluke voltmeter to check for continuity between the two. I've checked the headshell leads to the cartridge. All is well. Maybe it's the ground wire connection to the turntable itself? I've run out of options. Any ideas?