I tried sealing the slight gap between the cups and the pads with double sided tape using stock pleather and don't hear any difference at all. There is still no pressure/vacuum when I push and draw to check if there is a complete seal, so I thought the headphone itself isn't airtight around the driver. Theoretically a complete seal could improve sub bass performance AFAIK, but I haven't heard it.
The most notable improvement for bass with the Jerg mods on HE-500 was making "bass vents"
I encourage everyone to read through this guide made by Jerg/Modulor. Much thanks to them for going that far with it.
The Stock Denon pads have served me well for over a year now.
Looks like good contenders we have then are Denon, Audeze Vegan, and possibly Jerg/Modulor pads.
I should try the pleather pads then. The HE-6 is one of the most neutral headphones I've heard, but it is still slightly bright. Not HD800 bright, but one could hope for a bit more warmth. I would like to see how the pleather pads could make the HE-6 sound warmer, at the expense of soundstage.
Give them a try. Do this before removing velours though. Put your hands over the HE-6 grills. Notice the collapse in stage? That's the tradeoff, but it wont be that significant.