I got the HE-400 a few days back and i have the Fiio E17. In general, i am happy with the purchase, and i would definitly do it agian if placed in the same spot again.
I do not own the D600, so i cannot comment there.
From my personal expirience, the Fiio E17 drives the HE-400 very well (i had the chance to try the fiio E17 and then the Icon HDP desktop amp to compare, and i did not notice any quality diffrence in 20 minutes of testing, but do note that my previous setup was far worse, so there is a chance that, overtime, i would see the diffrence),
From my laptop, the HE-400 were untolerable to listen to at Gain 6, volume over 45, but this was from a CD and with Jetaudio maxed (and its the loudest program i have in my laptop).