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!