A couple of updates.
(1) Based on Lavorgna's description of how the iFi Micro USB power supply improved the performance of the Dragonfly, I'm probably going to grab one and try it with the Concero. No downside that I can see.
(2) As you might have guessed, the Concero is ruthlessly revealing of bad recordings. On the first two tracks of Craig Taborn's solo piano album "Avenging Angel" (96/24 FLAC download from HDTracks, transcoded to AIFF using Max), there are some really weird artifacts. I can't tell for certain whether there is some sort of "prepared piano" thing going on (a piece of paper on the soundboard, maybe?), or whether the piano was miked so close and so hot that you're actually hearing the vibration of the hammers. Whatever it is, the Concero doesn't hide it, not even a little bit. If you have a lot of 128k and 192k mp3 files, the Concero may not be for you, unless you're willing to spend to upgrade your music library.