I'd reckon that for listening to music, the Xonar STX is still one of the best sound cards! I have one myself, which I'm using in conjunction with my Sennheiser HD555 headphones & Corsair SP2500 speakers.
With my next rig, I'll probably buy a
Focusrite Scarlet 2i2 (USB) audio interface, just because I'm now into live music mixing at the church
The thing is, you do get better sound cards for studio purposes, where they have a neutral output, lower latencies, etc.
The Asus Xonar Essence STX does have a warmer (non-neutral) output, which you can easily change by swapping out its op-amps.
The only downside to the Xonar STX is that its only meant for stereo out, unless you're using S/PDIF.
About 10 days ago Asus did a press release about their new STX II and STX II 7.1
Unfortunately we still have to wait for reviews and its not available yet.
Here is a discussion about it:
http://www.head-fi.org/t/708756/asus-xonar-essence-stx-ii