You want one sure way to bring improvements to the XDSD? Plug it into micro iUSB - the upgrades are about 20-40% improvement depending on how resolving your headphones are.
What do I hear?
* More bass - deeper, tighter, and warmer.
* Less digital glare and grain, smoother.. I find myself playing my music much louder without fatigue.
* Quieter sections are more quiet, louder parts are louder - everything is more dynamic (could be related to previous point)
* Wider, more 3d soundstage - this is one obvious improvement,
* Treble is extended, but sounds purer - the extension is there as before but less fatiguing.
Overall, quite a massive improvement, certainly it's warmer and much less digital, it sounds like a serious piece of desktop kit. There are just my initial findings that I wanted to share. Can't wait to try it with my m2tech hiface 2 spdif interface and see if I get even more improvement