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I thought only the X-Fi had CMSS. This is good news if I am wrong because I'll have the Audigy 2 ZS Platinum in my hands next week.
To answer your question, I'm not sure. Try using google or search on these forums. Is there a PDF file on those sound cards that can tell you more about it?
I've never used the X-fi before, so can't say I've experienced the newer CMSS, but I think they improved upon CMSS (or CMSS-3D) so now it actually does a good job with headphones. I thought it sounded like ass for music and games on the A2...
CMSS 1/2 are on all Audigys. CMSS 1 is a standard type of upmixer, and it is also meant to be on for games to get smooth transitional audio in games. CMSS 2 adds some reverb and bass enhancements meant for movie playback. The upmix quality has always left something to be desired, and I used other upmixers, like foo_channel_mixer to upmix music for me. (Once the music is upmixed to 6-channel, CMSS ignores it, so I can have iton for games and not have it butcher my music playback)
The X-Fi incarnation of CMSS has been updated using technologies from MacroFX, Sensaura, and Aureal. The new upmixing techniques are now on par or better with other 3rd party types, like Neo:6, and definate improvements in HRTF surround with stereo output, like headphones.