If I were you, and were really serious about computer audio, I might try this:
http://www.htomega.com/clarohalo.html
Its about 200 bucks, but you are getting up to blu-ray quality audio, multi-channel surround, and actual RCA outputs if you want to line level to a stereo.
It also has dedicated amplification for headphones of up to 600 Ohms, though you have to move a jumper to enable it. It is based on Texas Instruments:
http://www.ti.com/lit/ug/slou169/slou169.pdf
Rated power is 2 watts at 32 Ohms. Not sure if it can do this at 600 Ohms, but this is more than enough power for most headsets. It is also about as much power as many really good headphone amps seem to throw out.
Basically, the card specs very well compared to DAC / AMP combos you can get for a similar price.