Thanks guys... I definitely did have the meter leads reversed. All readings are positive now...
So any thoughts on what could be causing this? The problem is there even with the gain at minimum and isn't affected by the gain setting at all. To add more mystery to this I have two other Millennia HV3C preamps at work, and my TLM170 has no problems with either of them - pad on or off. I had thought this might mean there is a problem with the AC power at home? Does that make sense? I have it running through a Monster power conditioner which is reading between 124 and 125 volts, and telling me the grounding is ok. I had this same problem on location too although I don't remember what the power conditioner said then.
Millennia was kind enough to replace the preamp when I called them about this issue and the replacement had the exact same issue, so the AC power is the only thing I can think of... Any thoughts?