In the last week I have had the time to sit down and spend some time comparing some of the DAC/amps which my family has been using for mobile listening. Specifically the DragonFly, CEnterence DACport, and a Fostex HP-P1. My reference point is a Lavry DA-11. The primary headphones I use when on the go are a pair of Etymotic HF3, but I also tried these for a these units with Sennheiser HD800, Stax SR-001, Thunderpants, Bose QC3, and Audio Technica ATH-M50S. None of these small units were able to really do justice to the HD800 or the Thunderpants. Both headphones could be driven to an adequate volume (pink noise with volume at max on the HD800 measured around 89db on the DACport and Dragon Fly), but they were held back by the lack of power. The Fostex could produce more more volume when on gain setting 2 & 3, but sounded like it was clipping. I didn't think to grab a waveform and see if it was. With all these units, dynamics weren't as dramatic when powered by a better amplifier, and a good bit of the transparency I love from the HD800 was missing.
I found the DragonFly and the DACport quite similar. In terms of practical power, I measure maximum volume using pink noise with the HD800 and the DACport could go 1.5db louder than the DragonFly. Compared to the DA-11, both had a somewhat flat sound stage. I noticed this with a number of tracks, but it really jumped out when listening to Chesky's new binaural recording. For example, when listening to Chesky's Dancing Flute & Drum through the DA-11 I felt like I could tell you where the walls were from the echoes. No such sense with any of the other DAC/amps.
When doing blind A/B between the DACport and DragonFly I had trouble telling them apart, though I seemed to slightly prefer the sound from the DACport. I think it was a bit fuller than the DragonFly, but it was a slight difference. I could always pick out the DA-11 and preferred it's sound: better clarity, control, and of course the better soundstage as mentioned above. While the DA-11 was clearly better, it wasn't approaching an order of magnitude better, suggestions that both the DACport and the DragonFly are good values but not giant slayers. If a friend asked which one to purchase, I would say either would be fine. The DACport has a slight edge on SQ and power, but the DragonFly is cheaper and smaller which can be a big plus for mobile use.
Alas, the Fostex HP-P1 wasn't able to keep up with either the DragonFly or the DACport. It's sound stage was even flatter. I also found it's bass not as tight, and it had less detail in the middles and the treble. I found the treble both a bit thin and bright. This unit clearly doesn't provide the same SQ / $$ of the other units though it was a clear improvement over the built in headphone out on our i-Devices and the Mac Air headphone jack. While I preferred the sound quality of the DragonFly and DACport to the HP-P1, the HP-P1 is incredible useful for those of us who carry around i-Devices, especially with the TOSlink output. This was very useful at a recent mini meet I was at.