Soundstage mostly, the hole in the middle. Found out I'm allergic to orthos and stats and most dynamics because they don't even bother to try to reproduce a realistic forward image. If you aren't sensitive to this issue, the HE-6 and Darkstar is a great combo. For me it's a dealbreaker, but most people don't even notice it, when it drives me crazy, so don't pay too much attention to my soundstage issues.
We have been working on improving this area with angled pads. Right now, the Audeze microsuede push the center image past your eyes. They also provide excellent bass and increased comfort. However, they only really work well in conjunction with the "Fuzzor" felt mod. Jerg is also working on an angled hifiman pad that I am interested in mimicking to see how they fare.
You're not past minor and reversible modifications are you?
Edited by brunk - 12/14/13 at 9:23pm