I'll add my impressions here, if you don't mind, BCPK.
FiiO D5 RMAA tests:
http://rmaa.elektrokrishna.com/index...&search_mode=f
The D5 is unfortunately not a great sound card for headphones. The wonky frequency response on the analog outputs and the very high IMD might not be audible, but the D5 hisses extremely much with any low-to-medium impedance phone. It is completely unusable with any multi-armature IEM, the hiss is almost as loud as the music.
Something like the 70 Ohm Koss KSC75 don't hiss with the D5, but the D5 is way too powerful even for them - the volume button at 0 is mute, and at 1 or 2 it is already too loud for regular listening. Using a 250 Ohm impedance adapter makes the D5 volume control usable with the KSC75... of course one could also change the volume in the audio player instead of on the sound card, but then you throw away the bits and lose a lot of dynamic range.
All in all, the D5 might be useful if you want to drive something between 300 Ohm Sennheiser HD6x0 and 600 Ohm Beyerdynamic DTxx0. Not so universal.
The line-out is volume-controlled, not fixed - and it actually works much better for IEMs than the headphone-out. The S/PDIF-out is quite good, as expected from a digital signal.
I'll post the
FiiO E10 RMAA results in another thread. The E10 is basically excellent, and if one's in need for a quality USB sound card-slash-headphone amp, save your bucks and get the E10 over the D5.