I currently use both the D1 and the DACport, though with low-impedance phones (Denon D2000, D5000, Grado HF2) and IEMs (Ety HF5). So while I can't comment on their performance with the hd600s and other high impedance phones, I do find that in my setup the DACport sounds audibly purer, more delicate, and a little more detailed than the D1. It images a little better, too, and does a noticeably better job of making instruments sound distinct from each other. In comparison, various instruments heard through the D1 sounds a little grayish and undifferentiated in timbre, as though made from the same material. And that's comparing the D1 heard through a good (glass) TOSlink cable—I find the USB input on the D1 pretty unremarkable, while the DACport's USB implementation seems far more thorough—the DACport's sonic background are also noticeably quieter than the D1s over either input, even at maximum gain. The DACport has pretty good impact, too, and hits hard when it needs to. The only areas where I prefer the D1 are bass—the DACport sounds tight but a little light—and drive: the D1 just has more, and sounds more authoritative when pushed. Most of the time, lately, I find myself plugging in the DACport, and the D1 isn't getting much use.
Hope this helps.