USB as the source from my computer. Needs to be able to power HE-400 Cans and have an RCA/XLR out so I can run some powered monitors.
Can spend up to $500.
Personally I love my Audio Gd NFB-10 (someone is selling one for under $500 currently in the classifieds) but it may be a bit overkill. The NFB-12.1 does everything you need (minus XLR outputs has RCA), the Matrix Mini-i, Asus Essence One, Cambridge Audio DACmagic Plus(a little over budget could get used in budget). There are many other all in ones that provide preamp/preout functions there are not many that have balanced outputs though (XLR).
I don't think any of these of these meet the criteria, he needs preout function.
Do you currently own an Essence STX or just weighting all options?
I'm a biased Xonar fan and Essence STX owner, so I would be happy as a clam, with STX, HE-400 & powered speakers.
Spending $500 for a DAC/Amp. is not something I've research on.
Is this just for music? or will you need surround sound for movies?
You did disable the motherboard's built in audio? in the Bios.
Have you tried getting the "Unified Xonar Drivers" from the website Brainbit.
For around $60 you could try replacing the 3 op-amps (operational amplifiers), get the LME49990 op-amps, off eBay, ships from Canada.
Try asking in this thread for amping the HE-400s
From someone who's owned both and moved on, the balanced out on the NFB-10 is noticeably better (more air, better dynamics) than NFB-12. Single ended is slightly better due to the beefier power supply and better components.