Well I finally had a chance to use these two headphones for a while.
ATH-AD900X vs ATH-A900x
Comfort and build - The ATH-AD900x are an extremely comfortable set of headphones. I used a rubber band on the wings to have a little more clamping effect and it worked like a charm. The pads are very soft and I could wear these for hours without any discomfort. In addition, these headphones are super light which means that the headphones don't fall off my head.
The biggest difference between the build of the ATH-A900X and the ATH-AD900x are the wings,pad material, and the weight. The wings on the ATH-A900x are a lot more flimsy and loose compared to the ATH-AD900x wings which also use a cloth like material on the wings unlike the foam on the A900X. Another problem with the wings being so loose is that the ATH-A900x is significantly heavier than the ath-ad900x. This means that the headphones will drop from your ears unless you have a very good seal(This is somewhat solved with a rubberband on the wings). The pads for the A900x are made of pleather in contrast to the cloth/fabric material of the ATH-AD900X.
The ATH-AD900x are noticeably superior in build and comfort, they are probably the most comfortable set of headphones I have ever used. The ATH-A900x ATH-A900x causes my neck to get tired after a few hours of use which gets quite annoying.
Sound - I'm not very good at analyzing sound, but both these headphones sound very good and I didn't find very big weaknesses for either. It's going to be pretty obvious that the ATH-AD900x excels in acoustical and classical genre, while the ATH-A900x excels in the more electronic and bass driven genres. I also love to listen to shoegaze, ambient, jazz and atmospheric songs on the ATH-AD900x as the larger soundstage allows the song to sound much more dreamy and vast.
The biggest problem I did have with the ATH-AD900x is that there is noticeable siblance for the headphones. The highs and hisses were at times piercing, but I help alleviate this problem by lowering all the 7hz frequency and higher by 3db. Another issue with the ATH-AD900x is the lack of impact on the bass, but it is an open over-ear headphone so its to be expected.
ATH-A900x sounds great with electronic, rap, and hip hop. The ATH-A900x definitely shines when I want to listen to something little more upbeat and fun like Bag Raiders. The impact on the bass is very tight and controlled unlike the ATH-AD900x where the lack of impact makes me feel like something is missing from the song.
I would have to say that the ATH-A900x is better all-around headphone and more fun to listen to than the ATH-AD900x which is much more genre specific. My favorite songs to listen to the A900x is:
Overall, if I were to choose to keep set of headphones I would probably choose the ATH-AD900x. I value the comfort of my headphones over having an all around headphone However, on a value standpoint ($99 for A900x, $187 for the AD900X) the ATH-A900x is definitely the much better bargain. I bought the A900x refurbished from buydig and got the AD900x new from JAPAN.