Two years ago, when I was looking to upgrade my aging ATH-A9X, I came across the AKG K550 and the ATH-A900X, both then newly-released, with several head-fiers reporting that they are tonally-similar headphones. Unable to decide which one to get, I bought both to do a comparison. I ended up keeping the K550 for myself and giving the A900X to my wife (who still uses it and loves it).
 
Below are my thoughts regarding how these two cans compare.
 

1. The A900X isn't a bad-looking headphone, but lacks K550's elegance. That said, comfort level is excellent and I in fact prefer it over the K550 in this department. The AT "winged" headband design makes putting them on and taking them off a breeze, and the angled drivers do not press against my ears even after my prolonged use (whereas the K550 causes discomfort).
2. One particular thing another reviewer complained about was the "poor quality" of the wings on the A900X. I looked into this issue, and did not personally find this to be true (relative to previous models). Below are my observations:
   1. On the old model A9X/7X/5X, the wings only pivot in one direction (let's call it the Y-axis), which is up and down.
   2. On the A900, AT implemented the "3D Wings" which pivot in two directions (X- and Y-). The way they achieved this is by making the wings themselves into a 2-piece design - there's an "outer rim" that pivots around the Y-axis (like the old wings), and an "inner piece" that pivots back and forth (X-axis).  This is actually a fairly intricate design and I imagine, harder to manufacture.
   3. On the A900X, AT simplified the "3D Wings" to ease manufacturing process while still retain pivots in both directions. The wings themselves are now back to a 1-piece design and pivots up and down (like the old wings on A9X), but the T-shaped joint where the wings are clipped onto the arms now pivots back and forth (in the older models, this joint is fixed). The joint on the new system makes the wings feel loose, giving the appearance of flimsiness, but having owned this headphone for two years I can attest that the construction quality on the wings are solid.
3. Material quality on the A900X is decent but not great. It's a step up from the A900 (which had the cheapest pleather possible and several plastic bits just look like sub-$100 headphones), but it's not at the same level as its older cousin, the A9X (which had supple protein leather earpads and wings). The earpads on the A900X is a durable-looking pleather that feels a bit on the hard side, and the wings remain fabric-covered like on the A900.
4. One nitpick: The stereo plug on the A900X is the exact same one AT has been using since the mid-90's (starting with the old ATH-AX series). It might have looked ok in the 90's, but looks a bit gaudy by today's more understated aesthetics.

 

1. The K550 is simply one of the most tastefully-designed headphone I've seen. The construction, materials, and appearance are all superb (my photos don't do them justice).
2. Comfort level is good but there are some nitpicks here... The earpads could be a bit deeper. The top of my ears do press against the drivers because the foam used in the earpads are extremely soft. The earcups are a bit stiff when it comes to pivoting, so they may not conform to the shape of your head without manual adjustment. The clamping force of the headband feels a bit loose, especially if you have small heads (and this is a headphone that already has sealing issues)
3. The plastic used for the signal cord, while fairly high-quality, is still more prone to tangle and deform in comparison to Audio-Technica's fabric-wrapped cords, which retain their shape better. I personally prefer AT's implementation.
4. One material nitpick... The headband padding is pasted onto the headband using basically a double-sided tape. Unfortunately the adhesive becomes a black goop as it ages and, in my case, actually started oozing out from the headband and making a mess on whatever surface I leave the headphone on. I ended up tearing out the padding, cleaned off all the adhesive, then glued it back using a glue gun.

 
 
Several Head-Fiers have claimed that the A900X and K550 are tonally similar, which was the reason I was interested in these two cans to begin with. They turned out to be very different-sounding cans, heh.

1. Treble: The A900X has a fairly clear and resolving treble. It appears to be tuned to offer sparkle without being offensive or sibilant (there's a dip at 7K which, I suspect, was intentionally introduced to reduce sibilance). Extension is absolutely brilliant, going full up to 20KHz. The only complaint I have is that it's noticeably grainy-sounding compared to the K550, whose treble is simply silky-smooth in texture.
2. Mid-Range: The A900X's mid-range is thicker, fuller, and more engaging than the K550. It is definitely a more engaging presentation (more "musical" if you will), but clarity suffers a bit here. The K550 in comparison is more clean-sounding.
3. Bass: The bass is my biggest gripe with the A900X - there's quite a bit of mid-bass bloat and the control isn't particularly tight. Bass extension isn't very good either - the rolloff starts at 50Hz, which means there's not much sub-bass compared to the excellent extension of the K550. Without any earpad mod, the A900X has a bit of a consumer sound to its lower end (I personally found that swapping the pads for the oval-shaped ones from the A1000X helps dial the bass down a bit)
4. Soundstage: The A900X has a very wide sound stage, likely owing to its excellently-resolving treble. Imaging is likewise excellent - I feel this is one area where it has a definite upper hand against the K550, whose soundstage - while large - feels a bit indistinct.

 
 

1. Treble: The treble is a bit of a problem area for the K550. Many users have complained about it being "peaky" or "sibilant". I don't find this to be the case, though the lower-treble is definitely too forward, causing the slightly unnatural treble presentation noted by many reviewers (a problem compounded by the thin-sounding mid-range. Both issues were addressed in the K553). I also wish there were less roll-off in the higher octaves, as the K550 could benefit from better extension (it's rolled off from 1.5KHz onward). Complaints aside, clarity is good and treble texture is superb - there is absolutely no grain on the K550.
2. Mid-Range: The mid-range on the K550 has excellent clarity, which I really love. However it is sometimes dominated by the lower-treble, which as I mentioned, is too forward. Additionally it could use some additional body - the overall presentation does sound a bit bright & thin.
3. Bass: The bass on K550 has excellent tightness and extension, however it could likewise benefit from some additional body. I personally do find K550's bass to be thin-sounding due to the lack of mid-bass. There's ample amount of sub-bass and upper-bass, but the mid-bass is intentionally recessed. A 2-3dB boost in the mid-bass would provide a more balanced bass presentation (exactly what the K553 did).
4. Soundstage: The K550 is well-known for its spacious soundstage, and I do agree it feels very wide and airy. However, I feel the imaging is a bit fuzzy and indistinct. Watching movies and playing games, it's much easier to tell where a sound is coming from on the A900X.

 
The A900X and K550 are both good headphones with distinct personalities. Neither are perfect, though. I would say the A900X is the more balanced-sounding of the two and more versatile when it comes to musical genres and other usage (games & movies).
 
The K550 offers better clarity and may be more desirable for analytical listening and editing tasks, but it's not as musical as the A900X and many will find its treble presentation to be problematic.

Admittedly the only reason I clicked in here was to see what "A9X" was/is - and I immediately thought of this cat:
http://www.audio-technica.co.jp/atj/show_model.php?modelId=228

Although I'm guessing the A9X aren't a 75,000 yen magnesium/titanium hybrid. Still cool to hear about old headphones.

Regarding your question about the K550 versus the "veiled" Senns (I had HD 580), I will say that they aren't similar imho, and that the "veil" is kind of over-stated in some contexts. They're (HD 580) smooth and gradually rolled-off on-top, but there isn't a "cloud" or "blur" over the treble like some dark headphones. The K550 were kind of hot in the upper-mids/lower-treble ime, and not as refined as the Sennheisers (or the K701).

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obobskivich said:

Admittedly the only reason I clicked in here was to see what "A9X" was/is - and I immediately thought of this cat:
http://www.audio-technica.co.jp/atj/show_model.php?modelId=228

Although I'm guessing the A9X aren't a 75,000 yen magnesium/titanium hybrid.

Still cool to hear about old headphones.

Regarding your question about the K550 versus the "veiled" Senns (I had HD 580), I will say that they aren't similar imho, and that the "veil" is kind of over-stated in some contexts. They're (HD 580) smooth and gradually rolled-off on-top, but there isn't a "cloud" or "blur" over the treble like some dark headphones. The K550 were kind of hot in the upper-mids/lower-treble ime, and not as refined as the Sennheisers (or the K701).

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The retail price for the ATH-A9X was $300, it was produced between 1997-2002 (I think). I had a more detailed review of it here: http://www.head-fi.org/forum/newestpost/649660
 
The housing is made of aluminium alloy, similar to the A900 and A900X, but it's not painted over like on the later models (many people mistook the A900 and A900X as having plastic housing). The ultra-polished mirror finish was a very unique and (imo) nice touch. Never seen anything like it before, and never seen anything like it since.
 
And thanks for explaining the sound signature of Sennheiser to me - I don't have much experience with them. Yes K550 does have a peak in the upper mids/lower-treble but that is typical of all dynamic headphones from what I've seen in frequency response graphs - I think the effect of this peak may be exaggerated on the K550 because the rest of the treble is so recessed.

great review- thanks. been a long time a900 fan, now have an a900x. Love the sound, but the wings feel very loose to me, with it had better earpads. I actually tried the higher quality imitation leather pads from the a900ti, it didnt help the a900x sound, neither did some WOW pads or jmoney pads. seems the a900x pads help its sound signature a lot. 

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ourfpshero said:

great review- thanks. been a long time a900 fan, now have an a900x. Love the sound, but the wings feel very loose to me, with it had better earpads. I actually tried the higher quality imitation leather pads from the a900ti, it didnt help the a900x sound, neither did some WOW pads or jmoney pads. seems the a900x pads help its sound signature a lot. 

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Personally I am interested to see if velvet pads may be able to reduce the A900X's bass quantity a bit so it's less droning.
 
The best example of A900X's bass issue would be the track Dreamers of the Day from Uncharted 3 - the beginning part of that song has a section that has this low, bassy rumble. On both the A9X and the K550, the rumble is controlled and in balanced proportion to everything else. On the A900X, the rumble completely drowns out everything else.
 
In non-bassy recordings, the A900X sounds full and great. In bassy recordings, the mid-bass bump often presents issues that are simply impossible to ignore.

Great review!
 
I actually just went through the exact same decision and ordered a K550 last night. I couldn't make heads or tails of how the two headphones compared (it and the A900X) because not only did people's opinions vary, but their entire perception of the headphones did too. It had both too little and just the right amount of bass, there was either too much treble or not enough, they needed an amp to sound good or they didn't....At the end of it all I just had to hear them for myself.
 
So I guess we'll see what happens when they arrive

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manyak said:

Great review!
 
I actually just went through the exact same decision and ordered a K550 last night. I couldn't make heads or tails of how the two headphones compared (it and the A900X) because not only did people's opinions vary, but their entire perception of the headphones did too. It had both too little and just the right amount of bass, there was either too much treble or not enough, they needed an amp to sound good or they didn't....At the end of it all I just had to hear them for myself.
 
So I guess we'll see what happens when they arrive

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Do you have the A900X?   Would love to hear your comparison as well.
 
While there is a great number of varying opinions, the observations I made concerning these two cans are consistent with several professional and user reviews (well, in the case of A900X, there was only one professional review - but overall user consensus here is it's fairly warm and full-bodied. I am not talking about comparison reviews; just reviews for the A900X). So it is definitely not "just me" - the very purpose of this review is to reach a definite conclusion regarding a few debates.  While it's impossible to objectively say "This one sounds better than the other one", it is possible to objectively say things like "This one sounds warmer than the other for certain" or "This one sounds thinner for certain".
 
The K550 is most definitely thinner-sounding compared to A900X. In fact, it may be a generally thin-sounding headphone: Read CantScareMe's review of K550, for example. Another seasoned Hi-Fier, MalVeauX, made similar observations as well:
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Originally Posted by MalVeauX

The K550 is colder, thinner, more analytical sounding. Drier. The A900X is warmer, more musical, bassier by a margin, less dry, more sparkle. To me, the K550 sounds like a tool for editing. The A900X sounds like something you listen to for enjoyment. Some people seem to think that dry neutral sound is "hifi" and to me, well, it's not. I like to enjoy music, not just enjoy the idea of listening to "flat".

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How people ever arrived at the conclusion that these two cans sound similar to one another is beyond me. Not to mention the claim that "K550 does everything A900X does and better" - that can't be further from the truth, because they sound nothing alike.
 
As aforementioned in my previous post, the A900X's bass is not only too bassy for me, but it offsets the overall balance and make certain songs sound very unnatural. If you have a pair of A900X, check out "Dreamers of the Day" from Uncharted 3 on them, then compare the bassy rumble in the beginning against how it's presented on a less bassy pair of cans. Between 31 seconds to 42 seconds, the droning gets pretty bad on the A900X; but the K550 just delivers is beautifully.
 

So, have you tried either one for positional audio in videogames?
I would love to hear a comparison in that aspect, between the a900x better highs vs the K550s 3d soundstage.

OP, you might like the creative labs WOW pads on the a900x. lowers the bass a little, definately more comfortable than stock pads

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jcrg said:

So, have you tried either one for positional audio in videogames?
I would love to hear a comparison in that aspect, between the a900x better highs vs the K550s 3d soundstage.

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I am assuming you play mostly FPS games? I don't game much anymore... need to think of a title that actually has good positional audio to test on.   Alternatively I can do some audio tests with movies that have well-mixed surround soundtracks. The effect should translate well to games. Will let you know.
 
As aforementioned though, I very much feel the A900X has stronger sound focus/positioning, and the additional detail in the treble helps in this respect. The K550 may lend a better sense of "space" when you game, which is great for ambiance, but it likely won't be as precise at pinpointing enemy locations.
 
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ourfpshero said:

OP, you might like the creative labs WOW pads on the a900x. lowers the bass a little, definately more comfortable than stock pads

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Last time I checked they were completely out of stock everywhere (I was thinking about getting them for the A9X). I'll consider it if I decide to keep the A900X at the end of the burn-in.
 

Great review and great counterpoint to the other review which I agree seems very unfair. If you want less bass, try the regular a900 as the x models were made to add bass. I myself am a basshead, and I think the bass is close to perfect. I wouldn't mind more sub-bass, but they definitely are able to reach down that far. And I think they're not so much warm as they are natural. Compared to my D600, vocals just seem so much realer. Like they're actually next to you singing. The D600 sound like the vocalists are singing into a microphone. A bit artificial. The reason I hesitate to call them warm is because they still retain an airy signature. The tones aren't overly dark and weighted down. Thanks for the review though. It's a shame that so many people read the other review and automatically dismiss them.

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bagheera said:

 
I am assuming you play mostly FPS games? I don't game much anymore... need to think of a title that actually has good positional audio to test on.   Alternatively I can do some audio tests with movies that have well-mixed surround soundtracks. The effect should translate well to games. Will let you know.
 
As aforementioned though, I very much feel the A900X has stronger sound focus/positioning, and the additional detail in the treble helps in this respect. The K550 may lend a better sense of "space" when you game, which is great for ambiance, but it likely won't be as precise at pinpointing enemy locations.
 
 
Last time I checked they were completely out of stock everywhere (I was thinking about getting them for the A9X). I'll consider it if I decide to keep the A900X at the end of the burn-in.
 

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I honestly feel that the A900Xs are better for positioning sounds in an horizontal plane, and the K550s are a bit better regarding sounds from above and below. But I'm just assuming, I really need an unbiased comparison in that aspect, because it'll be the main deciding factor in which one I'll buy.

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bagheera said:

Do you have the A900X?   Would love to hear your comparison as well.
 
While there is a great number of varying opinions, the observations I made concerning these two cans are consistent with several professional and user reviews (well, in the case of A900X, there was only one professional review - but overall user consensus here is it's fairly warm and full-bodied. I am not talking about comparison reviews; just reviews for the A900X). So it is definitely not "just me" - the very purpose of this review is to reach a definite conclusion regarding a few debates.  While it's impossible to objectively say "This one sounds better than the other one", it is possible to objectively say things like "This one sounds warmer than the other for certain" or "This one sounds thinner for certain".
 
The K550 is most definitely thinner-sounding compared to A900X. In fact, it may be a generally thin-sounding headphone: Read CantScareMe's review of K550, for example. Another seasoned Hi-Fier, MalVeauX, made similar observations as well:
 
How people ever arrived at the conclusion that these two cans sound similar to one another is beyond me. Not to mention the claim that "K550 does everything A900X does and better" - that can't be further from the truth, because they sound nothing alike.
 
As aforementioned in my previous post, the A900X's bass is not only too bassy for me, but it offsets the overall balance and make certain songs sound very unnatural. If you have a pair of A900X, check out "Dreamers of the Day" from Uncharted 3 on them, then compare the bassy rumble in the beginning against how it's presented on a less bassy pair of cans. Between 31 seconds to 42 seconds, the droning gets pretty bad on the A900X; but the K550 just delivers is beautifully.
 

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I wish I had them, I would love to do a comparison. Unfortunately I don't own anything that would make for a good A/B test. I mean, it's kind of pointless to put these up against HD800's or JH-16's. I'll definitely post my opinions when they arrive though
 
The problem is that even those objective things are still derived from a subjective perspective. Yes you can say that "headphone X is thinner than headphone Y", but then you can also say that "headphone X gives an accurate representation while headphone Y really colors the music". You're still perceiving and describing the same thing, but they've got two entirely different meanings. Most of the time you can decode everyone's combined opinions into what you'd actually hear with some sort of reasonable accuracy, but for these I just couldn't. Hell, even subjective opinions can be pretty objective around here. For example, "X is better than Y" can easily be considered objective if you're comparing UE8's to those free clip-ons airlines give you .
 
Have you tried them out with movies or video games at all by the way?

Great review.  
 
I saw you leading to this in other threads and am happy to see such a nice comparison.  So in terms of sound quality alone, you prefer K550.  Is this by a large margin?  Do you have any impressions of how these headphones compare to SRH840 or D2000?

Good Review Bagheera!
 
Question is... will you stay with both headphones?