**Update: I've seen frequency charts of these various headphones, and it appears there is a MINOR difference between the different K55X headphones. But the difference is so small IMO that I believe it to be negligible between each of the headphones in this series. The fact that AKG stated that they are really all the same (see below) as well further confirms how negligible the differences are.** Note: My Oppo HA-2 was used in this comparison. I've owned the AKG K553 Pro for some 4-6 months now, and have been VERY happy with it. The AKG K55X series is UNDOUBTEDLY "King" in my eyes (and to my ears). Despite the several other brands =407991]I've tried out since venturing into the world of Hi-Fi some 15-16 months ago, AKG is the only one that's met and/or exceeded my expectations on every front. But aside from all that, there's been this ongoing "debate" about what the difference actually is between the 3 wired versions of the K55X series (fixed wired, so not counting the K551 here)..........the AKG K550, AKG K550 MKII , and the K553 Pro (see pics below). Some say they sound different, some the same, but what is the deal with these 3 headphones?! I decided to investigate, at least 2/3rds of the way, sort of (just keep reading lol). The Facts So Far: The AKG K550 was marketed (seemingly) everywhere, where the AKG K550 MKII was marketed (I believe) only in the UK, and the K553 was marketed only in the U.S. (though of course with eBay you can buy all 3 from anywhere lol). All 3 of these headphones look identical (though see "Physical Differences" below), aside from which name is printed on the earcups. They all have the same premium gorgeous design and all 3 can be found for under $200 online. All 3 of these headphones have EXACTLY the same sound specs....... which you can see here (K550), here (K550 MKII), & here (K553). EVEN AKG says they're all the same!! lol But some people on here took out the speaker drivers from these headphones and found differences. I dunno about all that. I didn't do that so I can't verify anything. I just care about whether or not they look and especially if they sound the same. I used to own the AKG K550 a year and a half ago for a couple months. They were incredible. But I was swayed by someone on XDA to try another brand, the Sony MDR-1A, and so I went for them, selling my K550. Looking back now, I wouldn't have chosen the 1A over the K550 as I think I have a much better sense of what sound signature I prefer (neutral/natural with a BIG soundstage) than I did back then. But I liked the detachable cable (seriously AKG, make it detachable already!), and it seemed like the 1A got a little louder too. At the time my "green" ass didn't understand what a great amp (or now amp/DAC) can do to improve the sound, so I went with the one that got louder. But the smaller soundstage and more bassier sound (too bassy for me) of the 1A is why I wouldn't go back to them now. Though overall they were really great headphones actually, and I kept them for about 6-8 months until the Oppo PM-3 came along (which eventually the K553 replaced). So I went on eBay and I saw that some awesome vendor was selling the K550 MKII for only $142.99! It even came with the option to buy a 3-year warranty for only $20! So I said ok, why not get it and compare it to my K553? So I did. Showdown: The headphones arrived last Saturday and I decided I would do some burn-in on them just to loosen them up a little. So I hooked them up to my bedroom stereo for approximately 52 hours straight (aka, what I call "The White Noise Treatment"). From morning 'til night the volume was left up LOUD, but overnight I put it down to a normal level. I had the MKII with cups down so practically no sound got out. Then a couple days later I took them off the stereo and proceeded to compare them to my K553. Physical Differences.......... These headphones were identical, at least almost 100%. The MKII has its name printed on the vertical part just outside of the upper portion of the earcups, and vertically too. The K553 (and K550) both have their names printed on the "angled" part above there, and horizontally. Personally, I prefer the vertical approach of writing it out over horizontally, and that it's on the vertical outer part. You can see it better there than on the "angled" part, and it just looks cooler vertically written, to me anyway. Aside from that, I'd only noticed ONE other difference, this being that the K553 has slightly LESS padding on the underside of the headband than the K550 MKII (and I'm guessing K550 too) does! Why did AKG decide to do this? I don't know. Was it dumb? Yes. Did I notice a real difference in comfort? Not really, only slightly. I've fallen asleep many a times with the K553 on my head, so it was not a real issue for me. But I'd read at least one review online where the author was INFURIATED by this as he felt the difference in comfort was glaring. Just to further prove this observation, I let 2 other people I know compare the padding on the two, by sight & by touch (squeeze), and they both agreed COMPLETELY that the K550 MKII had more cushioning than the K553. So that's 3 very sure people saying this is so. I should add that this is a physical difference and observation, not a subjective one like we make all the time on here with music & sound, so it is indisputable. Sound Differences............. I compared only a handful of songs using both headphones since they were supposedly the same, and the extra tracks I only used the MKII for by itself were listened to in that way because I've had the K553 so long now I know EXACTLY how it sounds. I made sure to wear the headphones in EXACTLY the same way in order to assure a solid seal on my ears (using adjustment setting #5) too. Many people have complained about not getting enough bass from the K550, and a very wise person on here (shoutout to sludgeogre!) suggested that this may have been because people were not wearing them properly or the same way when comparing one to another. He also said that how "broken in" the earpads are can make a difference too, as a newer pair may not seal as well as a broken-in one. These are all factors I considered in my comparison. Conclusion: My final conclusion regarding these headphones is that THEY SOUND EXACTLY THE SAME!!! Every detail I heard on the K553, I also heard on the K550 MKII. Nothing more, nothing less. I chose all Hi-Res tracks to listen to, and these were all tracks which featured a lot of different sound, made use of the expanded soundstage, were both hard rock and acoustic rock songs, etc. I absolutely could NOT hear a difference, period. So in the end, as far as I can tell, these headphones sound the same. Since I like the look of the K550 MKII (and its box) slightly more than the K553's, plus it has the extra padding and a full 3-year warranty (vs. the 6 months I have left on the K553's), I decided to keep the MKII and am selling my K553. I am very happy I made this purchase and did this comparison as I feel I got more back than I put in. I HIGHLY recommend anyone interested in the K550 MKII to buy it from that eBay link above as I did with the 3-year warranty. That price is like less than HALF of what every other vendor on eBay (and even Amazon) is selling it for! But they only have so many, so act fast if you want it! As for the K550 (MKI), all I can say is though it's been some time, EVERYTHING about the K553 and K550 MKII sound-wise reminded me of how the K550 sounded, at least in memory. The big soundstage, the perfect neutral/natural sound, the perfect rendering of all 3 sound spectrums (bass/mids/highs) including the upper-end "sparkle" in the highs, etc. So based on memory alone (as well as the above-stated facts), if I had to bet, I would say it's extremely likely that the K550 would sound NO DIFFERENT to me than the K550 MKII and the K553 Pro do. I welcome all questions & comments. AKG K550: Spoiler: Warning: Spoiler! AKG K550 MKII: Spoiler: Warning: Spoiler! AKG K553 Pro: Spoiler: Warning: Spoiler!