It may have something to do with the fact that with the velours your ears are further from the driver. Some report that you get better bass when you lift the driver from your ears to create a bigger distance.
What about the instrument separation? Not sure if my ears are playing tricks on me but I get the feeling that wih velour I can hear individual instruments with better clarity, while the jergpad seems to give priority to vocals so background instruments seems slightly pushed back.
None of these are criticism of the mod, it's just a different tonality which some would prefer. Im still on the fence though. Im with you 100% about designing the perfect earpad. There's much more performance to be extracted from pad-fi, this mod proves it, we just need the perfect compromise if that's even possible.
Oh the decreased separation is because the pleather dust screen attenuates down a lot of treble, so background detail is not "over"-represented. Relatively it would sound like it has less separation than velours, but really it is just more subtle separation. I know because I have 2 sets of identical jergpads made right now, and one has pleather dust screen while the other has velour dust screen, and what you described is apparent between the two.