You're asking about two of the more elusive cans on Head-Fi... They're beautiful headphones but no one could give me any answers about how they sounded so I just went ahead and blindly purchased the A2000X.
Compared to the A900X, the A2000X has less bass. The sound in general is leaner and brighter. Very good treble extension, and a more refined, detailed midrange. I think someone here described the A2000X's sounding like the W5000 without the mid-range coloration, which is exactly how I feel about them. Soundstage is above average for a closed pair, between the A900X and the W5000.
Soundwise, it's not worth the $300-$500 for these cans IMO, which is exactly why I sold them 4 months later. The A900X is still the best bang for your buck. But they hold their own in terms of aesthetic value - they are uniquely gorgeous headphones.