CMSS vs Dolby Headphone is often an individual issue because neither is customized to a specific person's HRTF and uses a generic one based on some sort of averaging or modeling. That means one of them could work better for one person and the other could be better for a different person. You'll really have to hear it for yourself to find out.
Since you're on a budget you'll probably be ok with out a separate DAC. A separate amp might help though. If you're just running the Pro 80s from your X-Fi I'd imagine that it has a fairly high output impedance which will make the Pro 80's bass more boomy and overwhelming than it should be. Something like the O2 or the ODA if you prefer a more "normal" form factor with a low output impedance will show you how the Pro 80's bass is really supposed to sound and may reduce some of that messing around with the settings.