First of all, the best place to be looking for advice on DAC/Amps for headphones is
http://head-fi.org. Those guys know more than I ever will and you'll find tons of good reading. Most everything I talk about below, I learned either from head-fi posts or links to other sites I found there.
I've had two AUNE mini USB/DAC SE MkII for the last couple of years and recommend them highly. They're Chinese made and only available from ebay but don't let that put you off. The audio and build quality is high and the price is relatively cheap at less than $200 shipped. One drives a set of Sennheiser HD650 and the other originally a set of Sony MDR-7506 and now Audio/Technica ATM-H50. What's great about these is the high quality Burr Brown PCM1793 DAC and the ability to roll opamps to tune the sound to your liking. The stock opamp is a relatively run-of-the-mill Burr Brown OPA2134. For the HD650 and their warm, laid back, sound, I use an Analog Devices AD744 opamp to bring a little more life to the high end. For the Sony, bright sounding headphones, I use a Burr Brown OPA682 opamp to tame the high end a bit. Note these are subtle but noticeable changes. Driven via USB, the AUNE will accept only 16 bit 44/48 khz encodings but via the coax S/PDIF it'll accept 24/96 encodings (which sound glorious). I have both of these connected to computers via digital coax and drive them with Foobar 2000 on Windows and iTunes on OS X.
There's a successor now, the AUNE X1, which has a number of upgrades to the circuitry. I've not heard one but the review I link below thinks it's significantly better than the AUNE MK2 I have:
http://rockgrotto.proboards.com/index.cgi?board=talk&action=display&thread=7103
The Rockgrotto review of the AUNE MK2 is what convinced me to buy it in the first place and I wasn't disappointed. Note that there is also a matching AUNE X2 25w/channel power amp if you care to drive a small set of speakers as well. Price for the X1 seems to be about $200 shipped and both the X1 and X2 can be had together for around $300.
