I have the open ATH-AD900's and have owned them since about 2008. They are excellent headphones, but they don't fit everyone. I especially like the wide soundstage (particularly good for movies and gaming) and clear instrument separation. Highly recommended, especially at this price (I paid $300 imported from japan, 'cause they weren't available here in the US at the time)
The "X" (per A-T's product naming scheme) means it's an updated version of the (now discontinued) ATH-AD900 and ATH-A900. I have read they have better bass (the earlier models had somewhat less bass than what some people seem to prefer. Perfect for me, though.) The "D" means it's open.
My ATH-AD900's are very light, and are constructed of metal and plastic. The metal is very light: likely either aluminium or magnesium. I find them very comfortable, and have worn them for more than 16 hours at a time.
Since I sort-of want a decent closed headphone, I might try the A-900X, but I'm afraid the bass will be too high for me (and I'll have to EQ it down a little)
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Also, these are low impedance / high sensitivity headphones. External amplification is not required (but I always recommned a butter (gain=1) output stage on a DAC).
