OK, so I couldn't wait for my sample of Tempur to show up, especially not knowing if it was large enough to do the job. So, I cut up an old Tempur pillow to obtain the same material.
I initially cut it up into very thick pieces that could be scrunched up into the ear cup to provide the maximum amount of damping this material could provide while still allowing me to screw the wood ear cup to the assembly.
This initial pad looked about like this:
I sealed up the cups and listened. No go. This extreme amount of dampening with Tempur cuts into bass response too much. Yes, the headphone was more stable and more secure than ever, and no vibrations whatsoever, but there can be no denying the bass was attenuated too much, and slam and heft left too weak.
So I unsealed and removed a decent amount of Tempur, so that there was very little scrunching it into the ear pad; it fit pretty well, with just a modicum of scrunching. Sealed it up, tried again. Now we had bass roughly equivalent to the R10s. Tight as could be, but lacking in guts and extension, just like the R10.
So, finally, I opened them up one last time, took out another layer of Tempur, picked away at the center depression where the driver's butt fits in, and cut away at the outside edges, until I achieved a piece of Tempur that was roughly equivalent to the same size sponge I had in there earlier. The sponge, as you know if you've tried this already, although 1/2" thick, scrunches down considerably to fit into the ear cup, and retains its scrunched shape. The ideal amount of Tempur is roughly equivalent to the amount of sponge you had in there without the scrunching and compacting. Here is a pic of the finished Tempur damping by itself and in the cup:
The bottom line is the Tempur is an insanely better dampener than sponge, and it's quite possible to go too far with it and end up cutting out too much bass.
There are advantages of the right amount of Tempur over the right amount of sponge. The sponge mod seemed to have slightly reduced the amount of upper midrange and treble. Not so, the Tempur mod.
But there is a distinct disadvantage that made me ultimately reject Tempur. It seems to somehow muddy the bass response, and reduce upper bass warmth. It didn't sound as good as the sponge mod.
So, I removed the Tempur once again and simply filled the cups up with cotton, so I will try that next.
I may revisit the Tempur mod one last time, this time shaving it down to the very bare bones, to see if a super-thin layer might have all the advantages and none of the downsides.