I believe all of the Ultrasones share the same headband design (at least the three I have do) and the clamping force can actually be adjusted. There's an excellent description of how to do this here under the "Adjusting for Comfort" heading:
http://www.head-fi.org/a/ultrasone-owner-s-guide-the-missing-manual. My PRO-2900s didn't need this but I did lessen the clamping slightly on my HFI-2400s and quite a bit more on my DJ1s this way to get them all to a good comfort level.
The DJ1s are fairly comfortable over long periods of time for me now (I use them as my gaming headset) though I do notice they're there. The 2400s/2900s are incredibly comfortable for me, the larger velour pads are probably what makes the most difference there (I have the original DJ1s, not sure if the revised ones have the larger pads or not).