Long story short, Aureal and Creative got caught up in a legal battle, Aureal legally wins but is bankrupted from legal fees, Creative buys them up and buries their technology, freeing them of the biggest competitor in the sound card space they ever had. This also means that Aureal Vortex-based card owners got shafted hard in terms of driver support; there's some beta drivers for 2000/XP use, but nothing particularly refined. Better keep 'em to computers built to run Win98SE and games made for that OS.
From what I can tell, the Audigy and X-Fi cards support A3D 1.0 in the same sense that most non-Creative cards support EAX 1 and 2. A3D 2.0, when they introduced wavetracing (yes, they'd actually simulate how sounds reflect around a game environment, though it took a heavy toll on the CPUs of the era) and 3.0 apparently remain exclusive to the Vortex 2-based cards, though it's slightly more complicated in that Aureal also liked to refer to A3D as a sort of middleware. Damn confusing buzzwords...
Fortunately, most of the A3D-compliant games still seem to use other aspects of DirectSound hardware acceleration, so you're not completely hosed with non-Aureal cards. However, if they also offer standard DirectSound3D or EAX options, those are much easier to get full use out of (provided that you use a wrapper like ALchemy or GX2.5, of course).
The device you linked is basically a fancy front panel accessory for most X-Fi Titanium cards (but NOT the Titanium HD) and Auzentech's X-Fi Forte and HomeTheater HD. I think it's hideously expensive for what it offers and would suggest just using the rear outputs.
As for the Xonar Essence STX, I think its main benefit lies in that dedicated headphone amp. You could probably drive an HD 650 with it. (The Titanium HD allegedly lacks a discrete headphone amp built-in, but as a Stax user who's forced to use specialized external amps anyway, it's hardly a disadvantage.) For those that use Linux, I'm sure a C-Media-based card would also fare much, much better at just working than an X-Fi-based one. However, for playing games with hardware-accelerated audio support, the X-Fi Titanium HD and other higher-end models would fare better not just in terms of EAX 3/4/5 support, but for CMSS-3D Headphone outperforming Dolby Headphone in DirectSound3D and OpenAL titles, where it has more precise positioning information to work with and doesn't bother with emulating surround speakers in favor of emulating the game environment itself. (If they're newer games with XAudio2/FMOD/etc. software audio, then it's downmixed to 5.1/7.1 anyway and both work equally well.)
thanks for all your info!
so you think a Titanium HD with an external DAC/AMP would be a better option?
if so can you recommend any add-ons?
the STX is said to be great for music... would i notice a difference between that and the HD?