I see. How about pad depth? Could you try to put these vs HFM velour pads side-by-side and compare their relative thicknesses? One issue that arises with certain pads is they aren't thick enough, and people with wider ears tend to touch the drivers which hurts after a while.
Sadly I left the velour pads with the nude DT990 at the office. If it helps, I got the pleather ones to compare at home:
Right now the Beyerdynamics pad is actually slightly thicker than the pleather Hifiman pad, but they are known to get a bit thinner over time.. so what you explained might be possible.
If needed I can compare them again with the velour ones next week.
I think they are the same, when searching for the replacement parts on the German Beyerdynamic website it also suggested me the black one as fitting.
For anyone looking to do this mod, here is a picture of the back of the finished pad:
As you can see the elastic parts fit fine over the plastic ring from the HE-400 if you are patient enough and maybe revert to the hairdryer method as suggested by Beyerdynamic.
If you look closely at the holes you can see that this is how I inserted the foam pads into the thing. I started with one side and then used a small plastic toothpick to adjust the foam pad and keep it centered in the cup. I kept adjusting the pad through the holes in the plastic-ring to avoid any bulges. This is the most painful thing about the mod but should be done within a few minutes as well.
Edited by tomatolicious - 12/14/12 at 4:45pm