When I tested the Xonar DX I found them to be quite a bit noisier than the claimed 116dB SNR. At the time I thought that the original testing had been done under a very clean setting, and that the cards were picking up noise from my system. Then I put together a new system for someone and tested the card in a 'clean' system (no other peripherals) and found the same noise. The D2X was doing the same thing, particularly at the end of 'play-once' audio in games. You can be forgiving of vox noise, because you don't know the source, but clean game audio should remain clean.
The biggest problem I have with the cards is that they depend on Dolby for their precise 3D positioning algorithms. The cards also use OpenAL as one method, but in that case the precision is extremely lacking. In fact, in AH I would see position variations of about 30 degrees (estimated of course). I concluded that Dolby is a huge mistake for games like AH (verified since). The better X-Fi cards also use OpenAL (although not needed anymore) and even though they were not a whole lot better on noise offered the best 3D positioning available.
Enter the Z series, which has three flavors. The Z has a beam-forming mic, the Zx an audio control module, and the ZxR the ACM and daughter board for "Pro" level inputs. The ZxR offers the best SNR of all the cards at 124dB, which won me over immediately. The reason is simple. I use a 5.1 setup with speakers that are $160 each and while noise does not translate to distortion (necessarily) I still do not want to hear it when I am identifying an enemy tank location. Since most people will never need the DB Pro inputs the Zx is probably the better choice for most.
