I've owned both the AKG K550 MK2 and Sennheiser HD598 Cs, and currently still own an Audio Technica A900X and Fostex T40RP MK3 (with Denon pads and comfort strap). So you could say I've had very good experience with the headphones you've mentioned. With regards to the A990Z, I haven't heard this model myself. However, from what I've read about it, it appears to be a downgrade from the A900X. As I sold the AKG and Sennheiser and still own the Audio Technica and Fostex, that gives you bit of a clue to what I'd be recommending. I'll try and compare both the A900X and T40RP MK3 as best I can for you.
COMFORT: The first issue with the Audio Technica you’re going to have to consider is the 3D wing headband. If your head is on the smaller side, this headband design might not provide you with enough support. The standard ear pads on the A900X are large, but the inner circle for the ears is kinda small. I found it to become a tiny bit uncomfortable after a while around the ears if not fitted on your head quite right. There is a thread somewhere on this forum that has a list of good suitable replacement ear pads. Switching over to the T40RP, you MUST change the ear pads. The standard ear pads are terrible for comfort! They are so thin that they provide barely any padding. Fostex did this because the closer your ears are to the driver, the better the headphone sounds. Many people use Shure 1540 ear pads on the Tx0RP range, but I decided to use Denon ear pads on my T40RP. I did this because the Denon ear pads increase the comfort extremely, but still get thin enough to not harm the sound quality too much. Plus I believe this style of ear pad would work better on the closed version (the Denon ear pads are also far cheaper compared to the Shure's). The standard headband on the T40RP is fine, but once you fit a comfort strap it’s hard to go back (got mine from MrSpeakers). With both these headphones in standard form, the A900X is more comfortable. My standard A900X vs my T40RP with Denon pads and comfort strap, the Fostex wins for comfort. Once I get better ear pads for the A900X, I think the T40RP would still win, but only very slightly.
BUILD QUALITY: I’d easily say that the A900X feels more premium and better built than the T40RP MK3. The A900X uses metal for the ear cups and slightly nicer plastics, and is also Japanese built. The T40RP isn’t bad at all, it’s just not quite to the same standard. The A900X has an attached cable, but it’s a lovely flexible braided cable with a top quality screw on ¼ inch adapter (best headphone cable I’ve experienced so far). The Fostex comes with two detachable cables (3m ¼ inch cable and a short 3.5mm cable), these cables do the job but are nothing special.
SOUND: To be honest I’m not the best at describing sound, but I'll do my best. I would describe both headphones as having a very close to neutral sound signature, with slight deviations from this to make them more fun. I really enjoy both headphones for music, but probably feel that the T40RP is a tiny bit more special (maybe for being planar). The planar magnetic drivers also helps to give the T40RP a faster response. Both headphones have good bottom end, not being too weak or overdone, but the A900X might be better controlled with its bass. I wouldn’t describe either headphone as being dark or bright. For listening to podcasts or audiobooks, I would definitely prefer the A900X. If isolation is important to you, the A900X does a better job in this regard. The T40RP MK3 has vent holes where the ear pads fit onto the headphones, this doesn’t allow it to isolate quite as well. For more overall general varied use, I would pick the A900X. To select a winner here is very hard, which is why I have decided to just keep both. For more information about the sound, I’d recommend watching this review from metal571 which I pretty much completely agree with:
For the T40RP MK3, I’d recommend watching this video:
I’ll also let you know why I sold the AKG and Sennheiser. I didn’t find the K550 MK2 to be engaging which made it sound boring to me, left me feeling cold. It didn’t seem to do anything really wrong, I think it may have had its treble and bass a bit rolled off. For closed headphones, the HD598 Cs was just outclassed by the A900X, T40RP and even K550 MK2 for sound quality, it was really comfortable however.
I hope that helps audiophizile. If you have any other questions list them below and I’ll do my best to answer them. Best of luck with your choice.
