Unfortunately these tests confirm what I've observed with my Nexus 5 over the past month. Using RE-0 headphones (64 ohm, 100db/mW) I have to max out the volume most of the time. I can't detect signal clipping per se, but I can tell that it doesn't sound quite right with the volume control that high.
My stopgap solution has been to keep the volume control three clicks down from max (as recommended in the anandtech article) and use a Fiio E6 inline portable amplifier. It's certainly not ideal, but it's cheap and gets the job done.
I haven't gone through this thread to check, but my suspicion is that those who are happy with the audio quality on the Nexus 5 might be those with lower impedance and higher sensitivity headphones. Those with headphones that are tougher to drive have been less impressed.
So, since no USB DACs are currently working, the solutions are to mod the phone with a kernel that uses FauxSound or similar control of the amplifier, or to use an external amplifier, or to choose easier-to-drive headphones.
Edited by cthip - 12/13/13 at 9:47am