I got a chance to listen to the Concertinos recently (Abeit at a Tweeter). Unfortunatly I didn't have a good variety of CDs with me, but they sounded quite good.
They were too far apart to make a cohesive soundstage though, which wasn't good (They were around 20 feet apart or so, and not placed very well).
I don't think comparing them to the HD600s would be the best idea, as they have different sounds, at least in the positions they were in (On the speaker rack next to the wall).
For a small bookshelf, they had a lot of punch, and bass was surprisngly deep, but not boomy like the Klipsches I heard right before them. Moving them away from the wall wold have made it sound better though. The mids sounded slightly recessed, but that is about all I could ascertain and remember (Or perhaps is the Klipsch'es forward mids that made theese sound recessed).
They also sounded bright to me, but again this was comparing them to some cheap Klipsch bookshelves and the Celestions I got which are more dark then neutral, so again my perception could have been wrong. Silibance wasn't a problem however, and no sharp humps like the Klipsches had in the upper midbass.
Take my opinions with a big grain of salt, but in short I was impressed given the poor setup (And I had no intention of buying them, so the salesperson had no intention of moving them, he was a bit of an ass).
In short I was impressed and wish I could afford to get them, or another similar speaker. Definantly worth auditioning.