Discussion about the Korg PolySix synthesizer

I am having a terrible time tuning my PolySix.  I can get all 6 banks 
in tune with each other no problem.

I cannot get C0, C3, and C6 in tune at the same time.  To keep 
consistency, I checked each C on the same bank.

I've been through the calib procedure to no avail.

Can someone guide me to a potential point of failure?