Jerg, I hear exactly what you're talking about now.
I didn't notice it at first because I was focusing on the midrange. The mids to me sound too thick and wooly - which is the same issue I have with the HE-500 and LCD-2. The 400's are better than these, but it still lacks the clear, crisp airiness of my D7000's. To be cliche, it's like removing a veil when I switch to my Denon's. But to nail down the midrange issue, I switched to Adele's "Chasing Pavements" thinking her lower voice might help me figure it out. Sure enough, while the mids are definitely on the warm, thick side, I immediately heard the raspy, sibilant treble you have described. It's in the upper treble region (8kHz+) to my ears, and it's very annoying once you pick it out.