On another note, I've been asked to expand on my comments about "Positive Synergy with Pico and Denon D2000."
I've discussed elsewhere that my D2000 with APS v2 cable are great headphones, but they are a little dull or boring at times, like a closed HD600. A good amp can wake them up, and I've found that the Pico gives them some "Zing" that makes them a little more exciting, so I don't have to go to my Grados or Ultrasones for "exciting" when listening to ripped music on my Macbook. The bass is well controlled but is powerful enough to make the skin around my ears vibrate, the mids are rich and detailed, and the highs have a sparkle to them. Skylab also seemed to think the Denon have a synergy with the Pico, but he tried it with stock D5000, of which my APS re-cabled D2000 would be similar to that.
I think certain headphones are more picky about the amp you use, HD600 and D2000 included. My DarkVoice 336i that is so good with my Grados and Ultrasones didn't seem to give the HD600 and D2000 a lot of "Zing" - but my new Woo WA6 amp is burning-in to have a lot of energy with the Denon, once I got past 500-600 hours on the huge v-caps and blackgates capacitors.