My Marantz TT-15 and Parasound JC3 don't get along

I'm having compatibility issues between my Marantz TT-15s1 turntable and a Parasound JC3 preamp. I'm hoping someone on this forum might have some suggestions as to why.

Here's the story (in brief):

I had a Marantz TT-15s1 turntable and a Clearaudio Basic + preamp. They got along well and everything sounded good. Then my house got burglarized, and the thieves took the turntable and preamp. Thankfully, they left my NAD 375BEE integrated amp and speakers.

I liked the Marantz turntable, so I repurchased it. While I was happy with the Clearaudio Basic + preamp, I thought I'd upgrade to the Parasound JC3. I'd read all the glowing reviews, so I thought I'd try it out.

When I hooked up the new Marantz TT-15s1 turntable to the new Parasound JC3, I got a loud buzzing hum. I swapped out my interconnects, I tried grounding to my power conditioner, I made sure the settings on the JC3 were correct and nothing changed -- loud buzzing hum every time.

Since both units were new and untested, I wasn't sure where the problem was. I bought the turntable locally, so I brought the Marantz and Parasound to the shop and hooked everything up through a Dennon receiver the shop had on display. Same problem. We switched the interconnects again and the loud hum was still there.

When we ran the Marantz though the Dennon's photo preamp, the hum went away. We also tried the Marantz with a Moon 110LP phono preamp through the Dennon's aux input and, again, it worked fine.

So I thought there must be a problem with the Parasound JC3, right? I sent it back to Parasound. They ran it through its paces with various turntables -- with MC and MM cartridges -- and said they can't find anything wrong with it. (My Marantz TT-15s1 has a Clearaudio Virtuoso MM cartridge.)

My turntable works. The preamp works. They just don't work together. And I can't figure out why.

Any ideas?