Some more impressions on this... I have been using some JVC FXD80s out of the DF for the past couple week and supposedly these have a pretty similar signature to the newly released JVC on-ear S500s. I wasn't really hearing it, the FXD's seemed to be a bit too strong/loose in the bass and syruppy sweet in the top end, kinda smoothed over presentation... more to the fun side than accurate.
It didn't dawn on me to check out the specs for the phones... the S500s are 32 ohms, 106db/1mW, FXDs are 20/102. I had tried the DF -> O2 -> S500 and found that happily enough to be very similar to the sound to just the DF out. I'm no golden ears, it's similar enough that I would certainly fail a blind test. Now I just tried the DF -> O2 -> FXD80s and wow, I can more certainly see the family resemblance.
More interesting is that straight out of my MBA's headphone out I prefer the FXDs to the S500s. But overall I prefer the S500s out of the DF, it's a great pairing.
So output impedance is out of the question. But I seem to recall certain headphone amps work better at certain loads (usually when you see a spec sheet at different loads you'll see a bell curve of where they give the most current based on load). So perhaps for whatever reason the DF has trouble giving enough current at lower impedances. It's interesting that the O2 seems to add little to the sound with both my S500s and GR07s (50/105) but helps a tad to fill out my Thunderpants (50/98, not sure if the much larger drivers have something to do with it). It also seems to add a bit of control to the bottom end of my Ed8s (30/96) but not to a large degree.
As usual, YMMV, this is just my own interpretation of what I hear. I probably have at least a few hours of head-time now for each phone listed above with the DF, and some more like 20+, and the unit should certainly burned in enough at this point (TBH, I'm not hearing any obvious changes - maybe there were in the first day or two, but past that I'm dubious). If you have a bevy of phones at various impedances/sensitivities try it out for yourself. It's a pretty stellar device for what it is and if you have Amazon prime you can purchase, use for up to a month, and return with only losing the cost of return shipping. I can say for certain I like the S500s and GR07s straight out of the DF vs. out of my ODAC/O2, and that in itself is pretty remarkable.
Edited by bobeau - 8/20/12 at 1:18pm