Everything sounds great to me. I have the z-5500s hooked up to it via optical running DTS 5.1 and the surround is soooo much better than it was with onboard audio. I've done some tweaking in the card's settings. Music requires different settings than games but everything sounds fantastic!
Wait, that doesn't make sense. Granted the Xonar Essence DS is a nice card but when you output via optical as opposed to analog it's no longer doing any processing, that's purely done by the DAC that's present in the Z-5500 speaker control pod. You should have been able to obtain the same experience simply by outputting from your motherboard's optical output unless it doesn't have one.
this isnt entirely correct.
the card/mobo DSP chip still does processing, its just all in the digital domain now. the analog section of the card/mobo isnt used, but signal processing still takes place. ie, rate conversion, equalization, environmental, etc.
with optical out the final DAC is now in the receiver (or speakers), but the card/mobo still has a lot to do with it.
now, whether its entirely on chip or partially via software is another matter.