I'd say do non-fuzzy jergpads. Fuzzy ones had subbass rolloff and some bass quality issue I couldn't exactly pinpoint, so I got rid of the fuzzy layer. (Plus this fuzz is harsh in touch.)
When I attached them using duct tape on the sides instead of two-sided tape in the center, the hump went away. The hump got worse when I rubber glued the pads to the ring. I bet this is due to vibration from the driver conducting into pads and then ears.

Caveat: HE-500.

My focus is primarily on HE500s too, HE400s are just an afterthought really.

I think there is an unavoidable slight hump around 50 Hz if we use pleather earpads without the inserted foam rings, it may very well be linked to how shallow it is. But in terms of extension below 50 Hz, the pleathers (without inserted foam rings) just have no real extension to my ears unless I bond them to the cups.

Note that all my by-ear tests of extension etc are via SineGen.

Maybe distortion will still be high, but I'm darned sure that the FR extension with the pads Modulor will be sending to Marv for HE500s should be excellent.

I didn't notice anything weird that the velour cover shell did on the sound, it just removed some cupped-ness of the sound.

When I modded my D2000 to be open-back, I lifted the pads off the attachment plate/baffle. The locking pegs acted as "legs" then, so air vented under the entire pad (minus the pegs and velcro pieces I used to attach pads). For more information, here is the thread I made for the mod on HF:

http://www.head-fi.org/t/569094/open-back-mod-for-denon-d2000-5000-7000-fr-measurements-available-updated-11-1-2011

I got the idea from lmswjm, who actually sent purrin his version of the mod for measurements (available on this site). The primary difference between his mod and mine (besides D2000 vs D7000) was that he sealed that gap under the pads, and I don't believe he removed that small bit of paper(?) damping behind the driver or put a couple layers of foam in front of the drivers like I did (I was really trying to tame the peaks and ringing around 4-5KHz and 7-8KHz).

I made several changes after that post on HF. You can see that I never got around to updating it. The modding spiraled out of control, and before I knew it, I had a really messed up, almost unusable headphone. I had to toss them eventually. But, to get on with my point, see the differences vs my open-back mod and the stock D2000 (mine was not the latest version of the D2000, so I'm using Tyll's older measurements of the two). This was also before I went crazy with modding them into total crap...at least somewhat, I think. It's hard to remember:

Stock: http://www.innerfidelity.com/images/DenonAHD2000.pdf

Open-back mod: http://www.innerfidelity.com/images/DenonAHD2000DIYModifiedHans030390.pdf

Now, I'm not sure if it was the lifted, vented pads that caused it, but the changes in the bass on the HE400 here remind me of my D2000 mod. At the time, I thought it had improved the bass, cleaned it up, and kept the same, low extension as before. If the measurements were correct, my ears were fooling me. I actually did quite like how they sounded, though...very open, yet still could be powerful and intimate/close. I thought the upper mids and treble were much smoother. I liked it...but that's a side point. (I actually sent them to LFF to listen to...doubt he remembers it, haha, but that's off topic..sorry)

Hans did you test the extension with a sinewave generator or something similar, in that case? That's always been a trusty tool for me testing for extension, ringing, and clarity with headphones or modifications on headphones.