Chiming in...
My A900 pads were completely demolished, so I needed replacements(not wanting the same crappy pleather again...). So I ordered the A900X pads, which are a very durable, rather stiff/non-pliable pad that made the sound very dark/bassy.
I was very dissatisfied, especially after paying $40 for them from Japan. I was curious about what difference a stiffer pad would make, but I had no idea it would be so extreme... Fed up, I simply called Audio-Technica to ask for a new pair(after reading people on forums saying they called and got a free pair). I'm not sure if they make/sell the original A900 pleather pads anymore, but A-T sent me the AD900 velour pads for a mere $11. They fit, were easy to install(after I noticed the pad seam lines up with the bottom of the can), they are massively comfy(thank God), and best of all, I have my sound back!!! Well, most of the sound. Yes, these very soft velour pads decrease the bass from the original A900 pleather pads, but I would absolutely choose these over the A900X or A900 pads. Just adjust your EQ a bit.
Comparison of these 3 pads(on A900 phones)::
A900:
+ The most balanced sound
+ Oval shaped
+/- Medium comfort
+/- Most foam/padding (makes your head hot quick)
- Pleather...
A900X:
+ Most durable
- Circle shaped
- Very dark/bassy sound
- If you're 'sweatophobic', avoid these...
AD900:
+ Velour, velour, velour = comfort!
+ Oval shaped
- A bit more clean/sterile/trebley sound VS A900
- Low quality audio is very noticeable/annoying
**Update:
I found out why the A900 pads are special: they have a thin plastic seal/backplate directly behind the foam! Cut that out, stick it in the AD900 pads, then perhaps add a thin cut-out ring of semi-dense cloth to add some pad distance from the speakers, and you should be able to have your cake & eat it, too! The AD900 pads have a foam-rubber-like backplate/seal, which must be causing the sound leak. Although this does seem to improve the bass, the bright highs are still there. I would advise not buying more pads for hopefully better sound, and instead save up that money for HE-400's or something totally new.