Pros: Sound, Overall quality, Comfort, Closed-back but they don't sound like it, Bang for the buck
Cons: Non-removable cable, Cable is very long (3m), Can be hard to drive from small devices, not recommended for small heads because of sealing issues
This a review, as well as bit of a comparison to the Focal Spirit One, as those are the last headphones I owned and I absolutely loved them.
Man, where to start with these; right off the bat let me tell you that I absolutely love them. They are totally different from the Spirit One, and in a very positive way. When I first got the 550's, I was surprised at how heavy the box is. Now to anyone who thinks you get a lot of accessories with the K550's, I have to disappoint you. All you get are the headphones and a quarter inch adapter, the latter of which is gold plated and screws on top of your cable. Everything about the headphones exudes quality, which couldn't be said about my Spirit Ones, as their headband and joints are made only of plastic, and thus cracked under my big head. The 550's cable is not removable and it is 3 meters long. That's definetly something to keep in mind. Alternatively, you could go with K551, which have a detachable cable. Me, I just couldn't justify the hefty price difference between the two.
When you put the K550's on your head, there are two things you immediately notice (coming from Spirit Ones): Man, there things are huge! And; Man these things are comfortable! Seriously, comfort is insanely good, which once again can't be said for the S1.
I don't know if I would take these out in the public though, as they are quite big and the cable is looooooong for mobile use.
Enough of the chatter now, and let's get to the sound :)
I listen to music through the following setup:
-HP Envy 17 Laptop with FLAC's or Spotify- FiiO E07-K Andes USB DAC/amp (To bypass the HORRID beats audio on the laptop, that RUINS anything connected to it)- AKG K550
When I first started listening, I noticed that you absolutely need a good seal with these headphones, in order to enjoy them. For me, that's easy. But for anyone with small heads, and/or women, this might be a real issue. Try not to buy them blind like I did, and try them in a store to see if they fit. Once I found the perfect seal, I was amazed at how much bass there was. Compared to the Focal, which are said to have a lot of bass, I was really surprised because the AKG have more of it. The bass is deep and very well extended into the sub-bass range. It also never sounds boomy or muffled, nor is there too much of it.
Next, I wanna mention the soundstage: WOW, do they sound different than anything I've owned before, even though they are closed as well. They sound very similar to the K701, which I had the pleasure of testing. Instrument separation is excellent, and there is no "overlapping" of treble sounds, like I've noticed in the Focal. Mids are not as warm as the Focals' and overall, the K550 sound more detailed and technical, while the Spirit One are very warm and love vocals. Not to say that the AKG's don't, it's just a different kind of love :) . (The upper mids can sound a little (too) prominent at times)
Now for the highs, as they were the most notable change for me in comparison to the Focal; they are a lot more prominent and not rolled off, especially in the lower to mid-highs. They provide a nice amount of sparkle and excitement in my opinion, but sometimes at a cost: The 550's are kind of "unforgiving" when it comes to older recordings and lower bitrates. They will bring out every imperfection in the sound and can thus produce hissing and siblances. This was notable on older rock/metal recordings or such that weren't perfectly mastered, which is most of them. (Dream Theaters older albums are an example here).
Overall, I'd call the sound open, pleasent and enjoyable.
Should you buy these headphones? -Absolutely. That is, if you can find a good seal, and you don't plan taking these on your commute every morning. The great sound, quality and comfort, as well as the the awesome bang-for-buck ratio are reason enough to buy them. But for small heads, and a closed portable, I'd still favor the Spirit One.