Jsbail- A constant hum such as you are describing is most probably some sort of grounding/misconnection problem as it is happening with both cartridges. I used to have a P3 and recall that there is some way to ground it, but I can't recall exactly. I think it had something to do with connecting a wire to the arm at the attacment point underneath the table and then to ground, but I can't be sure. Sorry.
Anybody out there know about grounding a Rega? Maybe the Basis Brothers could describe how their RB300 is grounded?
The only other thing I can add is that I once had a similar problem and it turned out to be a wire not soldered well to the connector pin.
