Ok, I'm listening to the pico with my new to me HD600s with stock cable. It works very well, and I'd say it's a good option. I'm not the most critical of listeners, but listening to dire straits, it's got good imaging and depth, the dynamics and PRaT seem appropriate, and the sound is clear and forceful. I remain impressed by this amazing little device.
So I'll stand by what I said earlier, that I'd recommend the duet for fixed location or transportable use with a mac if you have use for full sized line outs or will use the line ins. Using the built in headphone amps, my preference is slightly towards the pico. I haven't done any level matching at this point, although I will later. I'll also feed the m3 with both and do a comparison. But really, you can't go wrong with either, so the obvious choice is high end IN and OUT vs OMG SMALL. Both sound great.