In light of recent discussions in this thread, I thought I'd post my experience personally performing a bunch of mods on my D2000. I did the mods one at a time, then reassembled the headphones and listened carefully between each mod.
Some of these mods made very audible changes in the sound of the headphones, but the magnitude of the changes depended on the specific mod. For example, dampening the ear cups produced an instantly audible tightening of the bass. However, I couldn't hear any change applying Dynamat to the drivers' rear magnets. And, the mods were not always for the better. I found that using Dynamat on the front of the driver assembly (perforated piece) caused a horrible spike in treble, apparently due to the metal covering of the Dynamat. After removing the Dynamat there, I replaced it with a different dampening material with a textured surface. That specific mod brought the midrange more forward, which I found desirable, but maybe you would not.
The great thing about the mods described in this thread is that Markl not only gave people step-by-step instructions on making his specific mods, he really taught us the process of making mods. So if you don't like the changes produced by a particular mod, you can undo it and try something different. And you can be inspired to come up with new mods. I added a thin piece of closed cell foam inside the headband, which made a huge improvement in comfort for me. (The headband stuffing was quite difficult and time consuming to do. Because there's a high risk of damaging the headband, I won't describe the details of that mod.)
Of course with these mods YMMV, and more importantly, YMCV (Your Mileage Can Vary). That is, you can tailor the mods to your personal tastes. The possible changes are limited only by your imagination for what you can apply where. As long as something is reversible, there's no harm done in trying. And even failed attempts help you learn what doesn't work. Find the earcup damping too constraining for the bass? Fine, yank it out and try smaller pieces until you get it just the way you like it. Or try some felt on the front of the drivers, just to hear how it sounds. Do whatever works for you, and don't be afraid to experiment. The materials are cheap, so you don't have much to lose.