Pros: Linear, smooth bass with great extension. Manages to sound fairly neutral while still maintaining good musicality.
Cons: Multiple issues in treble region. Mids are thin and lack body or presence.
Comparison Review: ATH-A9X, ATH-A900X, AKG K550
I have decided to update this review, not only because I've spent some more time with both the K550 and the A900X, but because I found frequency response measurements for both of them on Golden Ears. I know not everyone likes the way they measure, but since there are no professional reviews of A900X other than on DigitalVersus, and since you shouldn't compare measurements taken by different websites/reviewers, I felt this was the best course of action to take. I have also updated/adjusted several observations in the sound quality section, a lot of which I feel is much more quantifiable now that I have reliable graphs from the same source to look at.
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1. My Audio Preferences:
- I listen to mostly new age and orchestral pieces, so natural/realistic instrument presentation and strong vocal performance is important to me (good timbre).
- I like rich and lively sound presentation. So warm, pronounced mid-range; clear, sparkly, but unoffensive treble; and tight, controlled bass (that isn't overly loud).
- I dislike over-abundance of bass. My ears are very sensitive to bass; headphones with bass-emphasis make my ears uncomfortable very quickly and gives me a headache after a while (to put things into perspective, the Superlux HD668b is considered bass-heavy by me).
- I've been a long-time Audio-Technica owner, because their traditional sound signature is very much in-line with what I like: Rich, full-bodied, warm and engaging, with an emphasis on mids. The treble and bass presentation are typically pleasant and unoffensive on the AT's that I've tried. Aside from its narrow soundstage, my old ATH-A9X has always been my absolute favorite for music listening.
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2. Reason for Choices & Purchases:
While I love my ATH-A9X, I have been looking for a worthy upgrade that will improve upon its main weakness - the narrow soundstage (as well as having just a bit more substance in the bass region - the A9X is pretty bass-shy). After some research, I narrowed it down to two cans: The Audio-Technica's own ATH-A900X, and the AKG K550. Both of these are hailed as having very expansive soundstage for closed-cans, and both supposedly have a warm, "musical" presentation. They are reported to sound fairly similar to one another, according to several Head-Fi'ers.
While professional reviews of the AKG K550 are abundant in the English-speaking side of the world, reviews for the ATH-A900X were unfortunately very hard to find. The only two good reviews I found were the one on DigitalVersus (an European site) and one by our fellow Head-Fier, Mysteek. Impressed by the sound frequency measurements posted by DigitalVersus, I ordered a pair of A900X from a Japanese merchant on Amazon.
While waiting on the A900X to arrive, I noticed a few "comparison reviews" between it and the AKG K550 here on Head-Fi, most of which favored the K550. But opinions regarding K550's sound quality varies very drastically, and since sound is such a subjective experience, I decided the best way to determine for myself whether or not the K550 truly is superior in every way to the A900X is to listen to both of them.
So, having found a good deal for an used pair of K550 on Amazon, I placed my order.
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3. Objective of This Review:
The most detailed comparison review done for the A900X and K550 was written by Katun, whose neutrality I personally found questionable (not just the kind of word choices used in the review, but the attitude toward a few people who disagreed with his assessment).
That said, there's nothing more subjective than asking someone what "sounds good" when it comes to audio equipment - not only do people's tastes differ, but I suspect people's hearing ability also vary greatly - for example, I sometimes wonder if bassheads have poor bass perception, which is why they need their bass boosted that much... Okay that was a bit rude, for which I apologize. But in all seriousness, it is very possible that people who listen to bass-heavy music on bass-heavy equipments all the time suffer from hearing degradation, which affects their subjective listening assessments. This is why when I look at professional reviews, I like ones that are accompanied by graphs of actual measurements.
While I have no measuring equipments on hand, my hearing should be fairly "undamaged" (I don't typically listen to music very loud, and I am very sensitive to both sharp treble and loud bass), so my goal is to (hopefully) provide a neutral, unbiased assessment of the aforementioned headphones - not just by my listening experience alone, but by comparing them against the various reviews of them, both on the forum and on professional sites, and see if there are any consensus (to ensure that it's not "just me" that's arriving at these conclusions).
If you have either the A900X or the K550 on hand, and have a measuring equipment that can confirm my analysis, that would be great! For now, I will be attaching the frequency response graphs from Golden Ears in my comparison.
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4. Burn-in Status:
Both cans are fully burnt-in for this review (100+ hours); I now own both cans for over a month now.
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5. The Scoring System:
I will be using a scoring scheme to assign points to the "winner" in each category (between the A900X and K550). The score listed at the end of each category is the number of points assigned for that category only - the total number of points will be tallied at the end of the review. The old ATH-A9X will also be mentioned for comparison, but it will not be participating in the "match" since it's an old model that nobody likely owns in the U.S. and Europe anyway.
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6. The Review:
I. Packaging & Presentation:
Audio-Technica ATH-A900X: The A900X comes in a fairly subtle and classy paper box. The kind of cardboard used on the packaging isn't as thick and sturdy as those used on the AKG packaging, but it doesn't really hurt the presentation. What does hurt the presentation of the A900X a little is the plastic piece that held the headphone in place and kept the cups separated - it's made of thin and cheap plastic and looks a bit tacky in context of the otherwise excellent packaging.
AKG K550: The K550 has a stronger, more aesthetically-pleasing presentation in my opinion. The box is made of very heavy duty, thick cardboard that is surprisingly hefty. The plastic holder is covered by a layer of velvet finish, which lends a strong sense of elegance and class. While the packaging of the A900X is not shabby in any way, it lacks the same kind of presence K550's packaging has. Opening the K550 is akin to unwrapping a piece of expensive jewelry. This round goes to AKG.
Score: A900X - 0, K550 - 1
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II. Design, Aesthetics, Materials, & Craftsmanship:
- The A900X is not a pair of bad-looking cans by any means, but it also doesn't look extraordinary. It just doesn't stand out as something classy or luxurious. I don't love it, I don't hate it. It's better-looking than its predecessor, the ATH-A900, at least.
- I don't have any complaints about the craftsmanship of A900X - it's well-designed and constructed, and though I wished for a bit more metal than plastic on the frames, the plastic doesn't look or feel cheap. These looks like sturdy and durable cans.
- One particular thing that Katun complained about repeatedly was the "poor quality" of the wings on A900X. I took extra effort to look into this issue, and did not personally find his claims to be true. Below are my observations:
- On the old, old model like the A9X, the wings only pivot in one direction (let's call it the Y-axis), which is up and down.
- 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 ring" 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.
- 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, the joint is fixed). This new system is slightly looser, giving the appearance of flimsily-build wings, but I personally don't find the design or construction to be lacking in quality.
- That said, materials used on the A900X could have been better. I read in a Head-Fi post saying that A900X's stock earpads are the same as the A1000/A900Ti, but that is not true (because I just replaced my A9X pads with the A1000 pads, and I am 100% certain they are better quality than the A900X). The pleather used on the A900X earpads are of a cheaper, stiffer material - similar to the kind used on the Superlux HD668b. The headband's wings still use fabric padding, while my old ATH-A9X has pleather padding (which feels more classy). I feel like in the materials department, the A900X sits squarely between the A900 and the A9X - definitely a step up from A900, but not quite back to the glory days of A9X.
- One design nitpick - the audio plug used on the A900X is the exact same one used by the A9X; while it isn't bad-looking, it is a bit outdated in style, lacks elegance, and doesn't blend with the design of the A900X. Audio-Technica really needs to update this on their next product cycle for the Art Monitor series.
- In comparison to the A900X, the A9X is a lot more eye-catching. It's a bit "out there" in terms of design, but imagine seeing these cans back in the mid-90's - they must have looked sharp.
- In terms of craftsmanship, the A9X is on the same level as A900X. It's very sturdy, though to be fair, it has less moving parts (the cups don't pivot vertically, and the wings don't tilt in both directions)
- The materials used on the A9X is superior to the A900X. The stock earpads used the same kind of soft, supple pleather as the A1000/A900Ti pads. The padded wings are enclosed in thick, durable pleather as well (mine hasn't cracked after 10 years), which just looks better and blend in with the earpads when the headphones aren't in use.
- The fabric used on the audio cable is the same as on the A900X, but it's colored a deep brown to match the color of A9X's frame. It looks gorgeous, and more classy than A900X's black cord (which can easily be mistaken for a typical plastic cable if not examined closely)
- Oh my goodness... where to even start. I am sure I am not the only one who was blown away by the design and aesthetic value of these cans. The K550 is hands-down one of the most beautiful set of headphones I've ever laid my eyes on (right up there with my favorite AT woodies). All the lines just melt into one another... a true work of art. AKG wins out by a huge margin here.
- Craftsmanship on the K550 is good - that is, as far as I can tell (there are complaints about the swivel mechanism snapping easily because it's simply glued-on, and the headband seems to be very loose in general and lacks clamping force. This I observed as well). One thing I did immediately notice was that the swivel mechanisms are fairly stiff, and therefore the position of the cups tend to "stick", and often don't conform to the contours of the wearer's head automatically when worn. The headband is fairly loose and does not provide a lot of clamping force, which doesn't help the problem. This, I suspect, is the reason a lot of people have trouble achieving good seal on the K550. What the user needs to do is center the cups before putting them on, or at least adjust them after you put them on so they are flat against the sides of your head. Otherwise, I can find no issues with the craftsmanship here... I'll call this a tie between AKG and Audio-Technica.
- When it comes to materials, I cannot be happier with the K550. The cups and frames are almost entirely metal, so is the headband. The type of pleather used on the earcups and the headband are top-notch, and the type of padding used for them also feels very good - they feel like high-quality memory foam rather than the regular type of "cheap" foam used by most headphone earpads. Lastly, the audio plugs on the K550 are simply gorgeous - AKG spared no expense at making the K550 look and feel luxurious. AKG easily scores a victory here again.
- One material nitpick - 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 very well. Not going to discount AKG here though.
Score: A900X - 0, K550 - 2
(Two points awarded to AKG for winning "Aesthetics" and "Materials"; no points awarded to "Craftsmanship" due to a tie)
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III. Comfort & Isolation:
- While a lot of people have reported comfort issues concerning the A900X (specifically due to the wings system), I personally found it to be the more comfortable of the two. The wings do not put pressure on the top of my head at all - to the point I often forget that I am wearing anything. The headband also provides a very strong clamping force - these cans do not slide around on my head at all. Audio-Technica scores a win in this one (for me).
- Isolation is pretty good; these cans seal very easily and very well (thanks to strong clamping force).
- The A9X is actually more comfortable than the A900X in my opinion - it has less clamping force than the A900X, and is therefore more comfortable for prolonged use. The downforce provided by the wings are a bit stronger than that of A900X.
- Isolation is the same as A900X.
- The headband on the K550 does not provide much clamping force at all - and that's coming from someone with a big head (the headband settings are at 11 on both sides). While this makes the cups very comfortable around the ears, it also means most of the weight press down on the top of my head. While the headband is very well-padded on the K550, it does still exert enough pressure for me to constantly realize that I am wearing something on my head. The lack of clamping force actually allows the K550 to slide around slightly when I lower my head. Considering how many complaints regarding this the K550 has received in its own Appreciation Thread, I think it's safe to let Audio-Technica win this one.
- Due to aforementioned issue with the cup joints being too stiff (as well as lack of clamping force), isolation can be an issue if you don't be careful about achieving proper seal. Once fully sealed, however, the AKG seems to provide slightly better isolation. The difference isn't big, but I'll let AKG walk away with this one.
Score: A900X - 1, K550 - 1
(AT wins comfort, AKG wins isolation)
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IV. Sound Quality:
While most comparison reviews regard the A900X and K550 as similar-sounding (with K550 being better overall), I have to strongly disagree with these reviews. These two cans, to me, sound significantly different from one another - to the point where it's no longer apple-to-apple. It's most definitely apple-to-orange to my ears. This was actually rather shocking to me, because I expected very similar presentation with minute differences.
- Treble: Personally, I think the A900X easily wins the treble presentation. There are a lot more details in the treble compared to K550 - I think the main reason is because the treble is significantly rolled-off above 15KHz on the K550, while the A900X doesn't roll-off until close to 20KHz. I know human hearing isn't very sensitive above 16KHz or so, but the difference is definitely there. Another reason may be the K550's upper-mids/lower treble is way too bright and energetic, thus burying the details in the upper registers. A lot of subtle instrument textures come out easily on the A900X but are barely audible on the K550. But do not mistake A900X's treble as sharp or sibilant - because it's not. I actually hear a lot more sibilance on the K550 (details discussed under K550's section) - a good test track is Phil Collins' Two Worlds: hardly sibilant on A900X, very sharp and unpleasant on the K550. The overall treble presentation on the A900X is lively, sparkly, without being sharp or sibilant. Just the perfect balance for me - beautiful, beautiful treble.
- Mid-Range: It was interesting to see how many other comparison reviews claim that K550 has a warmer, more lively presentation, because the moment I tried out both cans I experienced the exact opposite. The mids on A900X are significantly more pronounced and warmer than those of the K550, especially in the lower-mids, where the K550 has a recess. This results in the A900X's sound having a lot more body (the perceived difference for me is bigger than the frequency response graphs may show). According to the frequency response graphs, while A900X's mids response is fairly linear (except for the down slope in the upper-mids), K550 has a dimple around 500Hz, and its overall mid-range response is more jagged / not as linear. The down slope in the upper-mids also helps A900X's lower- and mid-registers sound much more hefty and robust. Vocals sound more intimate, full, rich, and lively on the A900X. Orchestral instruments have both intimacy and presence. Personally, I prefer the mids on A900X; it sounds a lot more natural to me. Another point to Audio-Technica.
- Bass: The bass is my only issue with A900X's sound presentation. The extension in the sub-bass, while present, isn't pronounced like on the K550 (the one thing about K550 that I really really like). Also, there's a fairly wide peak between 50-250Hz, which makes music drony/boomy to my ears. Instruments that have a lot of both bass as well as lower-mids (such as cello) tends to have their mids drowned out by the bass, which is very unfortunate. I have tried EQing out the peak on the A900X and indeed, all the details in the lower-mids immediately came back. I'll admit that I am a bit of a purist in this regard and would prefer headphones to sound good without EQing or modding. Within the context of a review especially, I don't feel EQing is appropriate; these headphones simply are too bassy for my taste (without EQ), which is a shame because it has an otherwise perfectly balanced presentation in my opinion. It is possible to reduce the bass with different earpads though (see section at end of review).
- Soundstage: The soundstage on the A900X is very wide - especially for me, coming from the (in)famously narrow A9X. Sound positioning also feels better on the A900X than the K550 - sounds are more "focused" and easier to pinpoint (I had my wife test both cans and she actually pointed this out as one of her first observations, without any prior inputs from me). Additionally, the soundstage on the A900X feels more "layered" / has more depth (more details explained under K550 soundstage section). I think the point goes to Audio-Technica here.
- Instrument Separation: In my opinion, instrument separation is better on the A900X due to two reasons - more detailed upper-treble, and better sound positioning. Don't get me wrong, the K550 is a very clean-sounding headphone (I would argue quite a bit cleaner than the A900X), but I think its fuzzier sound stage and overly aggressive upper-mids/lower-treble hurt separation in busier tracks.
Frequency Response Graph of Audio-Technica ATH-A900X
- Treble: The A9X is actually more similar in this regard to the K550 than the A900X. It is very soft on the treble, especially in the upper region - the details are present, but you have to listen for them. Extension isn't naturally as good as on the K550 - though the A9X's drivers are capable (need EQ boost).
- Mid-Range: The mids on the A9X are some of the most beautiful I've heard (in my limited experience). It has that very old school Audio-Technica warmth and intimacy. I actually still like the mid-range on the A9X better than the A900X; it is fuller and more engaging, even though it doesn't have quite as much clarity - but nothing's perfect in every way.
- Bass: The A9X is fairly bass-shy, but that in itself is a pleasant quality for me since I get annoyed with bass-heavy cans very easily. The amount of bass present is more than sufficient to give orchestral instruments proper presence and impact. The quality of bass is also good - it is fast, tight, and clean. Once again, the drivers are capable - and can be EQ'd to provide appropriate bass for heavier genres like trance or techno.
- Soundstage: The A9X has one of the narrowest sound stage you'll ever hear, I promise you! I have no idea why it was made this way, but on these cans, vocalists often sound like they are singing right next to your ears; and orchestral pieces usually sound like it was performed in a small room with you standing right in the middle, amongst all the musicians. This makes certain pieces sound very intimate, but it's also a disservice for some music. That aside, sound has good focus and positioning is good.
- Instrument Separation: The A9X doesn't do particularly well here compared to either of the other two cans; partially due to the rolled-off treble & smooth mids. It's also possible that the older drivers on the A9X just don't offer the same kind of resolution as newer generation stuffs. That said, it doesn't really do all that bad.
- Ear Cups Placement: Okay... there's a lot of complaints in K550's appreciation thread regarding the clamping force and achieving proper seal affecting sound quality. While the K550 seals fine on my head without me having to hold the cups to my face, the K550's sound changes VERY drastically with cup positioning (as opposed to the A900X, which sounds pretty much the same regardless of whether you have the cups slightly forward, slightly back, or press them closer to your head). The K550 sounds very thin and distant if you don't press the cups to your head after you put them on; it becomes much warmer and more full-bodied if you press the cups in. If you move the cups forward, the mid-range is recessed but the bass comes out. If you wear the cups dead-center over your ears, the mids are more pronounced but the upper-mids get way too bright, and the bass gets buried. If you wear the cups back... well, forget it, you get the idea. It's very hard to gauge what these headphones are supposed to sound like, as per manufacturer intended - so I am going to go with what I think is the most pleasant-sounding wearing position, which is having the cups slightly forward (the rear of the cups rest gently against the outer edges of my ears).
- Treble: - Personally, I feel the treble is K550's weakest point. There are three issues with K550's treble presentation - the first of which is the significantly rolled-off upper-treble, which takes away quite a bit of subtle textures from instruments and vocals. The second is the overly bright upper-mids/lower-treble. As you can see in the frequency response graph, the area between 5.5-8KHz are significantly boosted on the K550, while on the A900X this region is deeply recessed. I think this is what's primarily responsible for the slightly unnatural, "plasticky" treble signature of K550 (several Head-Fi'ers attested to the unnatural treble; Tyll on InnerFidelity also mentioned it). While it might not be considered a sharp headphone in the grand scope of things, the K550 is a lot pickier with recordings than the A900X, and the excessive brightness can get fautiguing after a while, depending on what you are listening to (a lot of my J-Pop and trance collections are almost unlistenable on the K550; just way too busy in the upper-mids/lower-treble, but not enough bass and lower-mids to offset it). The last issue with K550 is that I actually find it fairly sibilance-prone - at least, much more so than on the A900X. And no, I don't believe the aforementioned peak in the upper-treble/lower-mids is to blame. I suspect the actual culprit for K550's sibilant tendencies lie in the small - but rather pronounced - peak at 12KHz, where the A900X already pushed its treble back down. Overall I find K550's treble a bit disappointing; even though it's not an issue in most music I listen to, when the problems do come out, it isn't very pleasant.
- Mid-Range: The mids on the K550 is interesting. Let's start by saying the K550 is relatively thin-sounding. Yes, the K550 is a thin-sounding headphone - this opinion has been voiced several times in various Head-Fi threads as well as professional reviews (Headfonia, Sound & Vision, just to name a few). It's not thin to the point of being unenjoyable, but compared to the A900X, the lack of upper-bass/lower-mids body is very apparent. As you can see in the frequency response graph, there is a dimple around 500Hz. While this is what, I think, lends the K550 its clean, near-black acoustic background, I really do feel it could use a bit more lower-mids presence. Percussive instruments and male vocals often have trouble achieving their full weight on the K550 (female vocals usually sound great, however). The second issue is of course, the aforementioned upper-mids/lower-treble peak, which makes a lot of orchestral instruments sound way too bright (the best example would be trumpets). I seem to remember Tyll on InnerFidelity mentioning that a downward slope between 1-5KHz is desirable - and the K550 indeed showed this downward slope in his measurements. However, the K550 displayed no such slope in Golden Ears' measurements - with the exception of a small dimple at 4KHz, the entire mid-range is fairly flat. The A900X on the other hand displays such a downward slope in the upper-mids. Not sure what's "better" in this case - but personally I find the A900X more natural-sounding and has better timbre.
- Bass: The bass presentation is definitely in K550's favor. I find K550's bass a lot cleaner-sounding, with much more audible extension. Looking at the frequency response graphs you can easily see why - the sub-bass region is significantly boosted whereas it is rolled off on the A900X. Since human hearing is less sensitive to sub-bass, this was probably a good tuning choice on AKG's part. I was surprised to see that K550 also has a mid-bass bump between 60-200Hz, because these cans sound a LOT less boomy than the A900X - which is a huge plus for me. The aforementioned dimple around 500Hz is probably the reason for the cleaner-sounding bass, though I am not sure. In any case, I like K550's bass presentation a lot more than that of A900X's.
- Soundstage: This is where I found things interesting. In terms of sheer sound stage size, I feel the K550 actually isn't quite as wide as the A900X. However, the K550 has a very interesting "spatial" quality to its sound stage. While sounds on the A900X seem to stretch out onto a 2-dimensional plane, the K550's sound stage sounds sort of airy and 3-dimensional. I suspect this has to do with whatever this "Real Image Engineering" AKG used to simulate the soundstage of open-backed headphones on these closed cans. Personally, I like this effect - however, sounds also feel a less focused compared to A900X, and positioning takes a hit as the result. Furthermore, the K550 seems to push sounds "back" within its soundstage - for example, a vocalist who sounds like she is singing only 1 meter away on the A900X will sound like she's 5 meters away when listened to on the K550. The effect is similar to listening to sounds through speakers (which, to my understanding, is one of the qualities of open-headphones?) Again, I do like this effect; it makes the presentation less claustrophobic and more relaxing. But it also means the K550 has trouble achieving intimacy or make things sound nearby. This issue is most apparent In movies: when objects fly past the viewer, on the A900X the proximity effect is very convincing, but not so much on the K550. This is what I meant by the A900X sounding more "layered" and has more depth - for example, listening to African vocals on the soundtrack for The Last Lions, both my wife and I can easily tell which vocalist is standing closer, and which is standing farther away. On the K550, this "depth positioning" is more difficult, because everything sounds farther away.
- Instrument Separation: As I mentioned in the A900X section, I feel the K550 actually doesn't do as good a job at separation, despite it being cleaner-sounding and having a blacker background (paradox!). The missing details in the upper registers and its tendency to get too bright and busy in the upper-mids/lower treble are the primary culprits - and the fuzzier sound stage contributes to it.
Frequency Response Graph of AKG K550
Score: A900X - 4, K550 - 1
(AT wins treble, mids, sound stage, separation. AKG wins bass)
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V. Total Scores & Final Verdict:
The Audio-Technica ATH-A900X and K550 both finished with 5 points - and believe me when I say this was not the intended result. I went into this review expecting two very similar-sounding pairs of cans, with K550 consistently out-classing the A900X in every way - and this was not the case at all. As you can see, the K550 won most of its points in aesthetics and materials, while A900X won most of its points in sound quality. This would indicate that A900X has superior sound quality overall.
However, this is not necessarily the case - because this is where the listener's subjectivity as well as needs come into play. Do I prefer listening to music on the A900X over the K550? Absolutely. But would I entrust the A900X with video-editing or audio-mixing work? Probably less so. The A900X does beautify the recording a bit more, in my opinion - and the K550's cleaner sound/black background are good qualities for sound mixing. One also needs to take overall sound balance into account - the A900X's heavy bass gives me a headache after a while; the K550, while being overly bright on occassions, doesn't have this issue. If I keep the volume down, the K550 is the less fautiguing of the two for long-term sessions.
In summary: If you like a rich, lively musical presentation that isn't overly exaggerated and sounds natural, the Audio-Technica ATH-A900X is the choice to go with. If you want a clean-sounding headphone with a fairly black background and fairly relaxed presentation (either for sound mixing or listening to slow music), then the K550 is for you. Personally I can't really recommend the K550 for general music/multimedia usage, especially not for the average user. It is simply too picky with recordings and genre.
Like I said before, apples and oranges.
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Usage in Video Games & Movie-Watching:
Since a lot of people were wondering how these two cans compare for usages outside of music, I did some testing with both cans. And to put it simply: The A900X is much, much better for entertainment usage.
ATH-A900X: My observations in music listening tests are confirmed once more - the A900X has much better sound focus, and therefore much better sound positioning in games & movies. The perceived difference is not small, either - one particular film I love to use for testing surround effects is the anime Sky Crawlers; it has one of the best surround sound mix I've heard in any film. The aerial battle sequences sound nothing short of spectacular, and listening to it on the A900X is like having an eargasm. The sound stage is very wide, and like I said the sound positioning is superb; you can hear all the planes circling around you and accurately pinpoint each of their individual location within the soundscape. Another movie that had very audible difference in sound positioning for me is Summer Wars, in a scene when a bunch of books were falling down from the shelves - you can hear the books land all around you on the A900X, but on the K550 the positioning just sound kind of fuzzy and indistinct Lastly, the boosted bass on the A900X, while an annoyance in some music, serves movies & games very well here. Car engines and airplanes have that proper weight and roar; gunshots and explosions have tremendous impact.
K550: The K550 is simply not that ideal for movies and video game usage. I am not saying they are bad for such purposes - I just watched Jurassic Park on them and they sounded fabulous. But the aforementioned "airy" and "3-dimensional' soundstage creates some issues with sound positioning/focus, and the "thin" mids on the K550 make things like car engines and machine gun fire sound overly bright and "hollow" (since there's no meaty bass or lower-mids to give these sound effects weight and presence, and the aggressive upper-mids/lower-treble contributes to this issue). To help you understand what a big difference this makes: A machine gun that goes "tuk-tuk-tuk-tuk-tuk" on the A900X sounds like "tak-tak-tak-tak-tak" on the K550.
So if you are looking for closed-back headphones for games & movies, the A900X is simply the far more ideal choice.
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ATH-A900X Earpads Replacement
I ended up doing a little tweak to the ATH-A900X since I wanted them to be less bassy, and it turned out the solution was staring me in the face. :)
I was looking at my ATH-A9X and the ATH-A900X side-by-side. My A9X has the A1000 pads installed (which is similar in shape and material to its stock pads; it has better pleather than the A900X pads). The A1000 pads has oval opening, while the A900X pads have round opening (which makes the holes a lot bigger).
Bigger holes = more trapped air = more bass.
Since the A9X is bass-shy, and the A900X is too bassy, I came to the sudden realization: Why didn't I think of swapping their pads???
The ATH-A9X with ATH-A900X earpads installed.
The ATH-A900X with ATH-A1000 earpads installed.
The result? Fantastic! :) The A900X is no longer droning whenever there's even the slightest bass present; the bass no longer bleeds into the mids and I can actually hear the sub-bass extension now. xD The A9X on the other hand received a bass boost - still far from as bassy as the A900X, but since the A9X doesn't even have D.A.D.S., to expect otherwise would be silly.
The A900X is very clearly superior in every way to the A9X - much better treble, much better bass, and much much better sound stage and positioning. Aside from the subpar stock earpads, I really can't complain.
So, A1000 pads on A900X - simple and highly recommended mod! Not only does it clean up the bass, but the pleather is a lot softer, more supple, and hugs the face better (which helps reduce sliding - especially if you have issues with the wings). So it's both a comfort & sound quality mod. :)