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?
I own the STX. I am very pleased with the DAC performance for my Monitors (I am pretty sure it uses one of the best Burson DAC's on the market) the headphone amp is a little lack luster for me HE-400's though.
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.