there is no "SB prefs" in ahi, its just the audiomodes list has expanded. you have to look for emu10k modes as mentioned before. if your soundblaster doesnt have the emu10k chip than probably it wont work as far as i know.
and - as i thought - the system itself supports 5.1 audio but theres no software at the moment that support it.
btw livemixer has a prefs for 5.1 audio, you can set the volume of the rear, center and bass. it has some sort of 3D support which i dont know if it works or not.

about linux, i have ubuntu installed, all sound works perfectly on the sblive: mp3s, videos, system sounds, every audio-related stuff. in fact , ubuntu has recognize the sblive at the install i guess, coz i havent got to bother with any prefs and configs - it just worked for the first time. well, it was a bit strange coz the onboard audio was completly ignored as i remember...