http://ixbtlabs.com/articles3/multimedia/asus-d2.html
"Both cards demonstrate the highest audio quality, it's very difficult to choose the best card. We haven't heard any differences in some test fragments. Others demonstrated a hardly noticeable difference in playback personality. The X-Fi demonstrates a tad more distinct attack, while the Xonar D2 provides a tad wider stereo base. I repeat, the differences are so measly that we had to switch between the devices many times to hear them. It's even more difficult to understand which card sounds closer to the original.
However, this situation is typical of modern High-End devices - it's very hard to hear the difference even with a high quality audio section. Manufacturers of popular devices have come close to reaching full potential of High-End converters. So you cannot expect a break-through in this field - the quality margin of audio components is disappearing. On the other hand, we can certainly recommend such cards as Xonar D2 and X-Fi Elite Pro for owners of high quality audio equipment, to say nothing of top active speakers."