Well there are 3 types of cmss3d. Headphone is the hrtf one. The other 2 are virtual and surround I believe.
But one thing I don't get is why would you need cmss3d for openal then if its mixing it in hrtf on its own. Doesn't make sense to me. That why I went looking and found the above info from the creative forums. That's the only explanation I can think of.