I have the HD-650's and had the Benchmark DAC. I also used the DAC-1 with my Krell KAV-400xi/Dynaudio Contour S3.4 speaker setup. FYI, Head-Fi.org has a ton of info about the headphone pairing with this DAC and others.
What I can tell you about my experience is that I didn't care for the sound. To me, it was harsh, analytical, forward and cold. My opinion applies both to the speaker setup and the DAC-1 directly driving the HD-650s. Sources I used were Sonos ZP-90, Apple Airport Express and my MacBook Pro digital out - admittedly not the greatest "transports" but I always used lossless files. I went to an Electrocompaniet EMC-1UP CD Player and wow, what a difference - warmer, definitely, but a much more realistic and enjoyable experience. I sold that and went to the ECD-1 DAC, which is basically the digital section of the EMC-1UP, and it still sounds great, but lacks the smallest bit of extra magic as compared the single-box CD player - probably attributable to jitter from the Sonos, cabling, god-knows-what - but it's still much better to my ears and in my system than the DAC1 ever was.
What I can tell you about my experience is that I didn't care for the sound. To me, it was harsh, analytical, forward and cold. My opinion applies both to the speaker setup and the DAC-1 directly driving the HD-650s. Sources I used were Sonos ZP-90, Apple Airport Express and my MacBook Pro digital out - admittedly not the greatest "transports" but I always used lossless files. I went to an Electrocompaniet EMC-1UP CD Player and wow, what a difference - warmer, definitely, but a much more realistic and enjoyable experience. I sold that and went to the ECD-1 DAC, which is basically the digital section of the EMC-1UP, and it still sounds great, but lacks the smallest bit of extra magic as compared the single-box CD player - probably attributable to jitter from the Sonos, cabling, god-knows-what - but it's still much better to my ears and in my system than the DAC1 ever was.