I have the HT21's and didn't really like the sound. They sound very thin and muffled, IMO, and they have that closed can honky-ness in the mids that I can't stand. I am not a fan of forward mids (in the vocal range), so some people may like them.
I ended up with the HA-S600's, which are listed as having carbon diaphragms, but I can't tell if they also have the nanotube drivers. They sound pretty good IMO, if you like a darker treble (I do). The best way to describe their sound would probably be "smooth", or "laid back" across the spectrum. I go to them if I am having ear fatigue from some of my other cans.
The problem with the S600's is that they are ported, so the bass gets farty, and generally sounds bad below the tuning frequency. Anyone know if they use the same drivers as the 400 series?