I'm using Asus Xonar DX and it sounds better on my speakers then on-board. I do have an inexpensive Logitech 5.1 Speaker System.
I'm connected via standard analogue speaker connector, not optical / digital.
It also sounded better (to me) compared to the Creative Labs X-Fi titanum I had on there before as well.
You can also test you sound card with this program
using a male to male wired connection.
Yeap I used to own the DX as well but fall in love with the ROG one. I have a #### 5.1 Logitech myself for the past 5-6 years