^^Oh, sorry weissglut, I didn't see your post until now, lol.
Anyway I got the DT880 pads from Hifiheadphones.co.uk, way cheaper!
Having used them for a week, I can say these pads (unmodded) sound reasonably good on the GMP 400. They provide a good tonal balance (in between the light and airy sound of the 41-6049 earpads and the bassy and dark sound of the stock ones), the sound is a bit more raw and maybe transparent and less smooth.
But these are still not presenting me the right sound...
The DT880 are less deep than the GMP ones and the hole is around 55-60mm.
EDIT: Apologize of topic. :)