I spend a lot of time in the amp forums, and the single biggest question is 'how can I inject some bass into these phones ?'. I think many of us are going about it the wrong way - wouldnt it be easier to just buy something from Ultrasone/Denon and take it from there ?
Personally, I found the coloration of the DJ1 lost its gloss fairly quickly, but (fwir) higher models in the Ultrasone range have a more balanced sound, but still retain that solid bass impact. It seems to me that spending $400 on something like the Pro 750 makes more sense than spending $500+ on a succession of amps that cant inject bass into headphones that simply werent designed to deliver 'thumping' levels of bass. Know thy headphones and thy sources - the rest will follow thee to bass heaven.
I dont really have a dog in this fight - I like the tight bass from my AD900/Grado phones - but this seems like a good place to raise the issue and offer a possible solution.