Korg Poly800/EX800 Users

I am playing in a band in which the guitar players are tuning down a 
half-step to accomodate the singer.
My other synths have tranpose buttons which the Poly 800 does not.
The tuning slider on the Poly 800 does not go down a full half-step. 
Is there another adjustment that can be made to get the Poly down an 
entire half-step.
Thanks in advance for any help.
Joe