I have had my Duet 2 since last Wednesday, and in comparison to my prior Apogee ONE, it is a considerable improvement as a DAC/headphone amp and as a DAC/preamp for my Speakers. As a headphone amp, the Duet 2 has considerably more clean output at much higher volumes, and is also much more controlled in the bass regions. Additionally, going from the ONE to the Duet 2 has brought a considerable amount of clarity to the reproduced sound through my speakers, both in terms of tonal clarity, and in terms of microdynamic detail and a considerably more three-dimensional soundstage. I am using the headphone out as a preamp going into my unbalanced speaker amp (using a custom 1/4" to RCA adapter cable), in order to bypass the breakout cable (which is not as detailed as the Duet 2's converters are capable of). Using the custom cable, I am getting a very detailed, three dimensional sound out of my speakers similar to my recabled K701 and recabled HD580 through the Duet 2. If I use my older interconnects through the breakout cable with adapters, the results are still good, but not quite as open or detailed. I also have found that the included USB cable isn't awful unlike the ONE's included cable, but upgrading it to a Kimber Kable CU B Bus usb cable (with the ferrite beads removed) has brought extra microdynamic and spatial detail through that the included cable did not provide. In terms of driving capability with my K701's and HD580's, the Duet 2 drives both very well, with the K701 being a good match with the Duet 2 as the Duet 2 has very deep, controlled bass, and very clean overall sound, and the HD580 being an even better match, due to the Duet 2's good control over the HD580 in the bass regions with a nice full, very detailed sound. My HD 598's aren't quite as good of a match with the Duet 2 as it is with my Corda Stepdance through the Duet 2, but the Duet 2 is still better with all three IMHO than the Corda Arietta, due to more detail and three-dimensional sound coming through the Duet 2 vs the more two-dimensional sound of the Arietta. Overall, I think the Duet 2 is a very good value, and is a considerable upgrade over the Apogee ONE. Additionally, the DAC of the Duet 2 is somewhat more impressive to me than the ADC side of the Duet 2, which is also a considerable upgrade over the ONE. The DAC simply sounds very detailed and clear no matter which output you use, but the headphone jack is more detailed than the breakout cable due to the more direct connection (and better cabling in my case).
--Eric
Edit: I forgot to mention that the ONE in comparison is less detailed, more two-dimensional and even somewhat muddy sounding as a headphone amp and as a preamp compared to the Duet 2, not to mention very weak as a headphone amp in overall power compared to the Duet 2.