EAX isn't surround sound...
The most simplistic explanation of what EAX is:
EAX: A collection of reverb and occlusion DSP effects
So, it does process sound depending on the source location in relation to the "viewpoint" (and objects in between) but it isn't a surround sound algorithm, it's just DSP
I think CMSS, set up properly as mentioned by Redo in this thread, sound better for games when using headphones. I think this is because you get the HRTF (head related transfer functions) DSP from the conversion from the game's discrete multichannel audio to the headphones two channels.
EAX is just good, I always have that on when available in a game.
CMSS (set up right) with decent headphones is better than CMSS disabled, and also better than 5.1 headphones.
I wouldn't use CMSS for stereo speakers though, it just doesn't seem to work, probably because of the inherent channel cross-talk in a room compared to the separate left/right of headphones (kinda like Dolby Headphone doesn't work on speakers, only on headphones) that's how I see (hear? lol) it anyway