After experiencing a few problems with some games and my Xonar D2, I spotted an Auzentech X-Fi Prelude 2nd hand. I thought it would be worth a try. I'm treating this as an audition and though I'd post some early impressions.
So far I've been using both soundcards unamped with Goldring DR50 (open), Goldring NS1000 (closed with active noise reduction) and Creative Aurvana Live! (closed) headphones. Have briefly tried Dolby Digital Live on both cards with speakers just to check it works.
First of all - they both work together in the same machine in Vista - sort of. The trouble is Alchemy doesn't seem to work with some games with a 2nd soundcard installed. It wouldn't allow me to enable EAX in Rome: Total War for example. Also tried this with an X-Fi USB Go! with the same result. For Alchemy to work, the Xonar needs to be disabled and the drivers uninstalled. The Prelude drivers can then be installed and Alchemy will work.
Secondly - gaming with headphones. Tried Bioshock on both. CMSS-3D headphone on the X-Fi and Dolby Headphone on the Xonar are very different. CMSS-3D headphone certainly gives a sense of direction but it doesn't do much for soundstage. Dolby headphone (especially DH3) gives a much more open feel, similar to listening to surround sound speakers. I could almost forget I was wearing headphones with Dolby Headphone. By comparison CMSS-3D headphone sounds very much 'in or close to head' to me. If anything, I would say CMSS-3D gives more directional accuracy behind the listener (i.e. rear left, centre and right) than Dolby Headphone. I could see it giving a real advantage in competitive FPS games. Dolby Headphone does a better job of tying in sounds to what's on the screen. CMSS3D headphone sounds a little too wide, which characters on screen sounding like they're off to the sides. I much prefer Dolby Headphone though, as it creates an artifical soundstage. This may be a reflection of the headphones I was using though. I think headphones with an already realistic soundstage could work better with CMSS-3D headphone than the ones I was using.
General sound quality with no dsp enabled is very close between the two. The Xonar is marginally better at driving headphones unamped. The headphones will be slightly louder at the same volume setting with the Xonar and it can drive them to a higher level without distorting.
For music - I tried listening to FLAC rips of a few CDs in Foobar2000, using ASIO drivers on both cards with no dsp enabled. They generally sounded very close. Apart from the above comment about the cards driving headphones, I'd be kidding myself if I claimed I could hear anything more than the placebo effect would allow.
I enjoy Dolby Headphone on the Xonar for music listening so I tried it against a combination of the 3rd party chanel mixer (set up to 2.0 or 2.1 with sound from all chanels diverted to subwoofer) and Dolby Headphone wrappers in foobar2000 on the Prelude. Software Dolby Headphone is simply no match for the Xonar. While the sound was certainly altered on the Xonar, I could not hear any loss of fidelity and all details present and correct, if a rather recessed compared to no dsp. Superficially Dolby Headphone on the Prelude gave the a similar sense of space and direction to the Xonar but some detail was clearly going missing and the timbre of instruments was sometimes altered, sounding less realistic.
Based on early impressions, it looks like I'll be keeping the Xonar. It's nice to have EAX back in a few games that weren't working with DS3D-GX though, so it's tempting to keep the Prelude long enough to play through them.
I can't find what suites me better. From what i've seen (maybe i'm wrong) the quality is the same and the difference is on db, Asus has more db. I play lot of games but usually with headphones, and i want really good sound for music and movies (with speakers) 
. What do u suggest from those two? 
