Sorry to dig up an old thread but this one appears to be the first one on google on the subject.
I searched around and everywhere i found people highly recommending the stx over the x-fi, so i splashed out for a shiny new stx and.. i'm very underwhelmed by it, i'm just going to sell my xonar and stick with my old x-fi as imo it sounds better than the xonar once you start using the crystalizer feature and bass booster, and the surround sound in gamer mode is fantastic in games.
On the xonar music and gaming lacks bass, bumping it up with the mixer makes all other frequencies become muddy sounding and crappy, whereas on the x-fi the bass boost is separated from the other channels and you can raise the bass as much as you like without effecting the other frequencies.
The dolby virtual speakers are just an echo effect.. alls it does is add a bit of echo to the sound, whereas with the x-fi it actually sounds like sound is coming from behind you.
the 7.1 virtual speaker on the xonar just does absolutely nothing for me.. alls it does is raise the volume, lowering it back down to normal it sounds exactly the same as without it.
Sample rate makes a tiny difference to how clear highs are in flacs, but the difference is really minor it's not even worth worrying about.
The power amp for the headset is apparently supposed to make the sound clearer and richer, alls it does is make it louder.
The flexbass setting turns the bass off. wut.
The environment effects are nice on the low setting, anything else is way over the top, but i'm not one for environment effects anyway.
The only positive thing i've found the xonar has over the x-fi is extremely low frequency bass has more of a rumble to it and is clearer, but it's not worth the sacrificing of other frequencies to hear it.
In the end, it's just not worth the price tag at all especially when the x-fi sounds better than it overall and is cheaper too.