I finally found the time to do some comparative listening between the DragonFly and Nuforce Icon HD tonight. A few notes:
-My main point of concern with the DF was its ability to project a sizable soundstage. Comparatively to the Nuforce, I'm happy to report that it earns a passing grade. The Icon can sound ever so slightly wider at times, but this is only noticeable in direct A-B testing. However, the DF does exhibit a greater sense of vertical space to my ears.
-The Nuforce sounds notably smoother and less dynamic than the DF. Individual notes don't necessarily sound weightier on the Icon, but instead thicker/wider/blunted.
-As implied above, tonal weight is a dead heat. I also don't perceive either as being noticeably warmer/brighter than the other. Both have (maybe) just a hint of pleasant, unobtrusive warmth.
-Due to the previously described smoothness of the Nuforce, the DF sounds greater detailed with a better sense of clarity and separation. Imaging/instrument placement seems more precise as well.
-The Nuforce has greater bass quantity. It's tough to definitively tell, but the DF may have slightly faster decay. (although, the tipped-up Icon is done very well)
-Although I don't currently have any high-impedance cans on hand, it's fairly apparent that the driving authority edge goes to the Nuforce. All things equal, it leaves plenty of room to crank up the volume. The DF is surprisingly good in this regard as well, (I'm at between 20 and 24 clicks out of 64, give or take, depending on the content) but with its external power supply the Icon's superiority is to be expected.
Overall, it comes down to needs and personal preference. It's certainly impressive that the DF has been engineered well enough to keep pace with the admirably performing Icon HD(P) given the form factor and lack of the need for a separate power source. The Nuforce would be my recommendation for those looking to drive power-hungry cans, those looking for some extra bottom-end presence, or who prefer a smoother "hi-fi" sound, but to my ears there's an overall greater sense of coherency with the DF due to its better perceived clarity, imaging, and depth of soundstage. Headphone synergy, as always, would be a factor and YMMV. For my current SQ needs and admiration for its overall convenience, the DF gets the nod.
Hours accrued: I'm unsure if any "burn in" has occurred or if it will yield a noticeable difference. I have yet to unplug the DF from my MacBook Pro since purchase and it has been constantly receiving power. (light turns red when the computer is asleep) Therefore, it's been ran in for about 72 hours so far.