I ordered the velour earpads at teh same time i purchased my HE 400s (from Headroom) but the pads were out of stock and backordered, so i listened to the 400s with the pleather pads for about two weeks. I was driving them with a Schiit Lyr, also new, and liked the combination immediately for all the reasons others have commented on in this thread: more air, great separation of instruments, terrific imaging, etc., etc. Overall, wonderfully musical. But. The sound also seemed darker than I'd expected and a little recessed, if that's the word, as if I were at a great performance but sitting a little further back in the house than I wanted to be.
Then the velour pads arrived. Installing them takes a little fiddling, but if I can figure it out, anyone can. For me, the difference was immediate, and dramatic. The darkness was gone, and suddenly I was sitting right where I wanted to be. I have no idea why this should be so, but the velour makes a seriously non-trivial difference, one of the cheapest and most effective tweaks I've ever heard. I should add that I'm using a Cayin CD player as a front end, and listen mostly mostly to classical, some opera, some jazz, some blues, zero rock.