QTR-Quadtone RIP

Hello everybody,
I obtain different results using the same curve on Mac or PC. The
problem is relative to Matte Black Channel. On the PC everything works
fine but on my Mac I get a bad gradient featuring an ugly abrupt step.
How can I fix that? I'm not the only guy having this issue, I really
need to get rid of that problem. If that can help I may post some
reference images. Bye.

Sebastiano Pitruzzello
http://phlog.it