[zowki]

M50 without cotton:

 
M50 with cotton:

 
I tried listening with and without cotton and I hear no difference whatsoever. Why is it even there?

 

[nickchen]

Such a thing is for damping purposes, and in order to minimize reflections. That is a minor effect, but if you were an experienced listener and had 2 pairs for direct A/B'ing at hand, you'd propably hear a small difference.