Okay so I wanted to get the Asus Xonar Essence STX when it came out because it boasts some of the best sound quality available in a sound card to date, yet I just recently purchased a HTIB setup that i was planning on upgrading to some new Polk speakers and such next summer when I free up some...
the DACs on your soundcard are much better than the ones that the Z5500 comes with, so using analog cables SHOULD give you better sound quality....the speakers aren't the best though so I can understand if you can't tell too much of a difference.