I think you hit the nail on the head with the J$ Beyer pads!
I just received mine today and I am liking them a lot. Now, to be fair, I don't have a good memory of what the velours sounded like as it took me a while to transfer the pads over (the clips can be difficult to clip on with these pads because the leather is a bit thicker on the tab side and it can interfere) but one thing I noticed right off the bat was the bass. Its not more bass but I find its better detailed and layered. Certain bass heavy songs I played now sound proper compared to the velours. With the velours on certain songs I was wanting a bit more detail or presence.
I also used one of my wife's nylon stockings to make a filters without any glue and its even more transparent than the velour nylon. THE DETAIL, WOW.
For people who have bought these, you can make filters without any glue. Just take the plastic ring and shove it in a nylon leg stocking. Don't put it all the way to the bottom, put it close to the bottom or half way. Enough so that you can grab both ends around the ring. Now, insert the ring onto the J$ Beyer pads backside. The pressure from the flap and the ring will be very snug that it will hold the nylon in place. All you have to do after that is cut the ends and trim the nylon closely all around the pad. That will expose the mounting rings and the nylon will not be going anywhere until you remove the ring from these pads. As i mentioned before, I had to actually pry the tabs a bit more open to get them to catch on the HE400 because it is now a bit thicker on the tab side of the ring but it did work and it mounts great.
One final note on comfort. These things are amazing. Fair above the velours on comfort, that is for sure.
Thanks for the great find on these pads jerg!