Distorted Headphone Signal!

Ive just got my first set of decks, so my gear aint great.

I have 2 x Numark TT200 and a Numark DM1002MKII mixer. And Behringer HPX2000 headphones.

The sound from my speakers is pretty slamming on both turntables.

But on my headphones, one turntable is fine, the other one the sound is really distorted, and only coming out of the right side of the headphones.

Also the gain levels are completely different even though they are the same model. Both bought from different sets though. (i bought one originally just to listen to records on, then i got the bug)

I've swapped the cartridges around and it had no effect so i know its not them. The tonearm maybe??

Does anyone have ANY ideas whats up??

Cheers in advance!!