Ya know, D2000 has done weird things to my mind. I have legit put it up on the classified a bunch of times now, only to back out and pull it back off. My intention was to "upgrade" and use some of the money from the D2000 sale to fund a TH-600.
But, of course, I just love the D2000 sound. Like you found, mods can make a big difference and once I got the alpha pads and wore them in a bit, I am a very satisfied customer.
If you go to the Fostex 600 homepage, on top of page 145, a guy by the name of "3X0" actually seems to prefer the D2000 over the newer Fostex. I think this is telling to what a great headphone Denon made. He likes it for the fullness of sound it creates, which I'm sure you understand what he means.
In any case, I still have the urge to try the Fostex, but I do not think I can do it at the expense of this D2000. I love it too much. I'm probably just going to save up so I can buy both.
TL;DR D2000 doesn't want to leave me, just yet!