See my signature: I own both the A900Ti and the AD900.
AD900:
This headphone has an ultra-wide soundstage, and has the most lush mids I've ever heard. That makes it an awesome headphone for vocal music. Jazz, especially female jazz, is simply amazing. The bass quantity might be lacking for some people, although bass quality is excellent. It goes very deep and is very well defined and textured, although the bass just doesn't have that much weight. The highs are very good. They extend quite far, but aren't fatiguing at all. And something else I shouldn't forget to mention: this a fast headphone!
A900Ti:
This special, limited edition of the regular A900 is a beast. I've never heard the A900, but I've read many comparisons between the two where people say the A900Ti is in another league. I'd like to believe so. Bass is excellent, period. Have you ever listened to either Amon Tobin or Burial? Both artists know sub bass. My A900Ti handles that perfectly. I can actually feel the bass and sub bass, yet it's all still very well controlled. The highs extend very far, too. Definitely further than with my AD900, but again, it's not fatiguing at all. The mids are very neutral, so vocals in the music don't stand out as they do with the AD900. The soundstage is okay at best. Definitely a lot smaller than that of the AD900, but that's to be expected, as this is a closed headphone.
Conclusion:
All in all, the AD900 is best suited for listening to natural music (non-electronic) with vocals. It also excels at classical music. The A900Ti, on the other hand, is best suited for electronic music and rock. I love both these headphones, but feel that the A900Ti is the better one. Of these two, it's certainly the one that wins the PRaT-award. I don't know the exact ratio, but my A900Ti probably gets 75% of head time, while my AD900 gets 25%. When at home, that is, as my SE530 gets 100% when commuting. But that, children, is another story.