I can personally attest to the quality difference between the two different RAMDACs. The red XK handles the conversion a lot better than the X and XP. I had convergence issues in B&W mode with the X and XP, but the XK works perfectly.

I'd say overall the XK is just simply a better card, and it also has the latest firmware. I'm tempted to swap the firmwares around and see if that does anything.