Phono Pre-Amp stopped working out of no where

Hello,
I had a Pro-Ject Tubebox DS phono pre-amp I bought new. Worked fine, I probably used it less than 20 hours. I upgraded my amp and so I sold this pre-amp on eBay (mistake). It was in mint condition.

The seller received it, said he was a musician and claimed it didn't work at all. He sent it back and in deed it doesn't work at all now.

• It powers up, the light is on and everything but neither channel is putting out any sound what so ever. I even tried a different power cable and no luck.
• I checked the jumpers on the back, they seem fine and set according to spec.
• I plugged my turntable directly into my amp, it works. I plugged another device into the unbalance line in the amp and it works. So it's not my turntable or that input channel on my integrated amp.
• I removed the top shields on the tubes of the pre-amp, and the tubes look like they're in solid. I tried taking them out but they're rather difficult so I was nervous about pull them out.
• The shipping box and inside box are all pristine. No dents or anything. No water damage. The unit itself seems perfect too cosmetically.

Any ideas or thoughts on what could have gone wrong? Did the buyer plug this in to a live channel and blow something? Could the tube be loose? Do you think this could be easily fixed?

I'm a bit frustrated as this was a like-new amp and this seller now is claiming I tried cheating him and I'm stuck with a broken $400 phono-preamp. I'd love to pass it along to someone else to enjoy.

Thanks,
Manish