I've just replaced the velour pads with the stock pleather, and to be honest I prefer the pleather ones.
The HE400s probably dont sound as engaging as with the velour but I find their timbre more accurate. The presentation is also a bit more distant and relaxed, which I don't mind at all. Ah, they are also more comfortable! Am I the only one preferring the pleather?
I much prefer the Velours to the stock pleathers. Mostly for comfort though, the sound with the pleathers I felt was a bit warmer, with the velours a bit more clear. I can't wear the pleathers for more then 30 mins without ear sweat though, the velours I can wear for hours and hours with no issue at all.