The Headroom Micro DAC is decent enough. I like it as I find it quite transparent, but the USB interface only goes to 16 bit / 44.1kHz. So if you plan on going higher than CD quality you may want to look elsewhere.
The CEntrance DACport LX would be another great option, but it runs just outside of your budget. It runs $299. It is a dedicated DAC sans an amp which should maximize your audio quality through your rig.
Let me know how you like the Asgard D5000 combo. I use the D2000 with my m903 and really enjoy the musicality.