halp 2nd opinions

I recently bought my first tube phono preamp off audiogon, a Pro-ject Tube Box II. I connected everything correctly as per mine and the previous owner's undrestanding, but I get a distinct hum in the left channel. I switch the tubes around and the hum remains in the left. I have it connect to a power conditioner so I assume it isn't AC hum. He says he never heard such a hum, and i'm inclined to believe him. The only difference is that he sent me the unit with the stock tubes that he never used (said he used Brimar CV4004 tubes and Telefunken E188CC). Anyone have any ideas what it might be? I figured it wasn't the tubes if the hum didn't change channels. Everything is quiet when I don't use the phono preamp as well. I've also tried hooking the preamp into other amps and get the same result.

Thanks for your opinions!
Switch your right and left channel connections to see if the hum changes sides, if it does, the hum may originate from your cartridge headshell connection.
I'll check tonight, I think i tried this before. So if it is the cartridge connection, what should I do? Because when I connect the TT to the phono preamp on my old Sony receiver everything is peachy.
Make sure ALL your connections are tight and secure.
Also, could you take it to a friends?
Go from there...
If everything is "peachy" when you connect to a different phono pre-amp, then its probably the Pro-Ject tube box. Make sure of all connections, then replace the tubes in the tube box with known good ones. If it still hums, make arrangements to send it back, or have it looked at by a qualified tech.
McFarland you are right about the headshell connection! I got a new turntable (MMF 5) and the pro-ject preamp works fine with it. I guess my old TT really wasn't up to snuff. Thanks for everyone's input!