The velour is thick and uses more foam overall I think, so there is more opportunity to trap reflections. I think the foam is also less dense and lacking the memory effect. I'll be switching back to the velours for a renewed sonic impression in awhile, but I am willing to argue that so far, the velours have proved to have better long term comfort and less listening fatigue (especially EQ'd). I keep thinking about two things with the HE-400s - what would angled drivers sound like? Also, what would pleathers with a cylindrical, rather than a donut shape, sound like with preforations that ran in the back and up the sides? I can picture them made in layers, like a modern bed. Pourus foam in half, with a memory foam 'topper'. Abandon the spacers and make them a little bigger.
They again the mild mods I have done have already brought out most of the best in the HE-400s - nice fat warmish sound and still lots of detail. I will do some sweeps eventually to see if I can figure out the ringing frequencies a little more clearly.