Interesting... now I'm not sure if you know this or not, but wouldn't the CPU perform most of the sound processing, then send the result to the sound card which can then further alter/improve/post-process this before finally outputting in the format of your choice(depending on speaker setup)?
I most certainly have noticed an improvement in positional audio, which I chalk all of that up to X-Fi "CMSS-3D". I can now tell if a sound is coming from behind, the front, the side and even above or below thanks to this tech. Sound card post-processing is definitely at work.
Now I'm curious about how exactly a sound file goes from raw/compressed file to sound engine, to CPU, to sound card, to DAC/receiver(optional) to speakers.