the only thing on the asus dx is i wish it had optical output if i were gonna plug in my astros...
Are you talking about the optical out because you have an Astro Mixamp? Because the Astro A40s can plug directly in to the Asus Xonar DX, and still get surround sound emulation that IMO is better than Dolby Headphone since it also simulates environmental effects and the height of the sound source. With headphones, mind you.
If you have the Mixamp, that can basically serve as an external DAC and soundcard if your motherboard's built in audio can supply 5.1 Surround through the Optical out port. The advantage of the Mixamp is that it can serve double duty between a gaming console and PC, but it doesn't offer as many features as an internal sound card.
Anyway, hope you enjoy your results.