Korg Poly800/EX800 Users

Dang didn't think the email was going to be posted directly so let me give you some specifics. I just bought a Poly 800 with Slayer Mod off ebay in (supposedly) perfect condition. I received it the other day and everything seemed great to the untrained ear but while playing with a friend we noticed the pitch was off. For every patch, the pitch is transposed up a minor third (ie every time I play a C regardless of octave a D# is heard in the relative octave). I've made sure it's not the detune parameter and the tuning slider only controls a half step so I'm wondering if something could've happened when the person modded the synth. Any information would be excellent.

There is an internal trimpot for tuning, I had one that had drifted quite far in time as well, tuning it sorted it and its been stable since.