Do a web search for "PAX Sound Blaster Z".
If your 8 channel set up is good quality, I would give the Xonar a try, no headphones surround emulation will beat the real thing, I have a good 5.1 setup and these days I use headphones only for online gaming, no more. I have nothing bad to say about the Soundblaster Z in terms of sound quality, quite the contrary. Nevertheless, I did have to fiddle with a lot of different drivers and swapping slots on the mobo until I managed to get it stable.
There's a thread here in which you will be able to see potential problems you will be facing:
http://forums.creative.com/showthread.php?t=699420
In the end, if you decide to go for it and you can get it running well on your computer, it will be worth the effort and current price.
Cheers.