I’ve been looking for a straight comparison between the original K550 and K550 Mkii / k553, so I decide to perform an A/B test with my old AKG K551 (2012, which is supposed to sound like the original K550) and my new K550 Mkii (2015 edition). I was surprised about how much significant are the difference between these two models... First, the aesthetics: my old K551 have slightly thicker earpads and deeper and slightly larger ear hole. Honestly, it doesn’t affect the comfort that much, but some people with larger ears than mine may prefer K551 earpads. I found the clamping force a bit higher on the K550 Mkii out of the box, but until it’s easy to bend the headband, it’s not a big issue. The headband padding seems a little bit more cushioned on the K550 Mkii too. I first noticed that the K550 Mkii sounds significantly louder than the K551 at the same volume level. Since these two are 32 Ohm of impedance, Could it be the wire and / or the different earpads? (I don’t know…). K551 have a short, thin and twisty cable (with a mic/3 button remote) in comparison with the long and larger cable of the K550 Mkii... Soundwise, the bass have definitely more impact with the K550 Mkii, and it seems just a little bit less thight/dry (more fluid) than the K551. However, bass extension is wonderful on both models (since they’re absolutely not for bassheads). The K550 Mkii seems have more dynamics in general. The mids seems totally uncolored, smooth and flat on the K551, but it can sound a little bit thin/dull and laid back on some tracks. Mids of the K550 Mkii are more textured (especially male vocals) and brings more emotion to the vocals and guitars in general, without being colored. They also sound more forward and energetic on the K550 Mkii than the K551, which is smoother and airy. The highs sound a bit more realistic to me on the K550 Mkii. The cymbals tend to sound thin and cardboard-like and sometimes indistinct on the K551. The “S” sound also a bit more sibilant to me on the K551. Both have well extended highs, but they sound much peaky and slightly more “artificial” on the K551. Soundstage is definitely wider on the K551, which sounds more like an open-back pair. On the other hand, K550 Mkii sounds more like a very good closed-back (with better dynamics). It brings more energy, fun and emotion to the music, but the narrower soundstage makes them sound more “in your face”. Overall, K551 sound more airy, but a little bit more thinner to my ears because of the slight lack of texture of the low-end and the sometimes sibilant / indistinct highs bothers me. I think the better bass presence and the richer highs of the K550 Mkii gives them a bit more of texture, bringing more pleasure overall. Bass: K551 Very good extension, very thight and dry, less presence K550 Mkii Very good extension, a tad more thump and better presence Midrange: K551 Uncolored and smooth, but a tad cold and distant (more clinical) K550 Mkii More energetic, more presence and more texture Treble: K551 Well extended. Could sometimes sound a tad thin, indistinct and artificial (cardboard). More peaky treble K550 Mkii Very well extended. Sounds more real (cymbals). No sibilance. Very good Soundstage: K551 Wider, airier, better instrument separation K550 Mkii More congested (“in your face”) Sound signature: K551 Pretty close to “midrange oriented” open-back headphones. Airy, smooth, relax and flat. A little bit laid back and on the thin side K550 Mkii More dynamic “closed-back type” sound. Rich sound, mostly flat and well extended all across the frequency range Comfort: K551 Slightly less cushioning in the headband. Earcups are thicker, with less softness. Earhole are larger and deeper K550 Mkii Maybe a tad bit more cushioning in the headband. Earcups are a tad thinner, softer and more shallow Overall, those two are probably the more accurate and flat closed-back pair I’ve heard. You can’t go wrong with either... Hope this comparison could help!