Hm, well. My personal suggestion to everyone looking for audio equipment is to start off "cheap" (don't break the bank purchasing something that others _love_ when you may very well have a different opinion on it). Given that you have a nice set of headphones and a relatively nice AMP, I would suggest looking at the $200-$500 price range for a DAC.
My experience, personally, is rather limited, but I can say that the Modi multibit is a pretty nice DAC (at least in comparison to the soundcards I have used in the past). The Modi, however, will not fit the current matte black aesthetic you are going for.
One that I would definitely have my eye on is the Jotunheim. At $600 you can get a pre-amp, with multibit DAC module, a black finish, and (likely) fairly good quality. I haven't had a chance to play with a Jotunheim, but it is definitely one I considered a lot when I was looking for my initial AMP/DAC solution; I just decided to save a bit of money at the time (which was the right decision for me).