Top Line: If you're looking for a K/Q 70x that doesn't leak like a sieve, IMO these ain't it.
For those of you who are somewhat old audiophiles, the K/Q 70x are the Martin Logan CLS and the K550s are the Wilson Watt2/Puppy2. I happened to love them both, but they're pretty different. What this means...
Amplification Requirements - On my iBasso D3, my K702 @ 3 o'clock = my K550 at 11:00. Generally speaking, it's easy to clip the K702, I can't imagine clipping the K550 at any reasonable volume. The K702 verges on unlistenable without at least a halfway decent portable amp that's been warmed up, at which point they open up like a flower and really come alive. The K550 sounds pretty good straight out of your iDevice headphone jack, but benefit substantially from good amplification, so they're MUCH more flexible.
Comfort & Fit - The K550 is significantly more comfortable (even though the K702 isn't bad itself), but the K702 is MUCH easier to fit correctly as you just have to put them on. You have to fiddle with the K550 a bit to get the seal right, and as previously mentioned in this thread numerous times, the seal is all-important. It wasn't to hard for me as I have a largish head, yet I was using the smallest size setting and they still felt a little loose. They designed these cans for giants. Once you do get them on right don't talk, chew, or shake your head vigorously. If you do, you'll lose the seal a bit.
Edit: If you wear glasses while you listen, the K550 is NOT for you. While they are more comfortable with glasses than the K702, you just can't get a good seal with glasses on - and this utterly ruins the K550 sound. This is a bit of a bummer for those of us who stare at screens all day and thus appreciate our Gunnar Optiks.
For Busta Rhymes' Do It to Death on ELE - While it's basically unlistenable on the K702 it's completely enjoyable on the K550. The bass is deep, tight, and nuanced on the K550 whereas it's just largely absent on the K702. Any more bass from the K550 and it wouldn't be right - you'd be entering into Beats territory. In fact, I think AKG may have voiced these things so that a Beats person wouldn't be immediately turned off. That sounds bad, but they pulled it off marvelously without ruining the AKG neutrality.
For STP's Down on No.4 - Here's where you can see the full tradeoff and how while very different there is some degree of equivalency between these two headphones. Going from K702 to K550 you lose virtually all the 3D grit on the guitar riffs and as a result they sound about half as "grungey," ...but you gain this thickness to the bass and mid-bass background that adds a layer of richness that you wouldn't have missed with the K702 until you heard the K550. It gives a tonal depth to the K550 that the K702 just doesn't have and as a result, the backdrop to performances are "blacker/darker."
For Solas' Newry Highwayman on Solas - The K550 just can't hang with the K702 here as the track isn't particularly bassy and the K702 is capable of delivering all that's needed on the low end with superior definition and detail across the whole spectrum. This definition and detail brings everything alive and into a 3D space that the K550 presents incredibly admirably for a closed back headphone, but not nearly as well as the K702. As a result the guitar, vocals, and even drum are just more rounded, palpable, and dynamically edgy on the K702. You get a much better sense of the attack, sustain, and decay of each note in the broad midrange and therefore the PRAT is better.
Gripes - The cable is too long and needs to be detachable. While on the topic of the cable, please note that my K702s were wearing a custom Blue Dragon cable for my comparison. Some of you will believe that affected my results and some won't. I obviously do, but it's been a long time since I've heard the K702 with the stock cable and I have no idea where it is so I can't comment on the issue.
Bottom Line: My very first impression was K702 with "loudness" turned on, but after a few hours and a plethora of different tracks I developed a much more nuanced view. As for house sound, at times (particularly with rock) I felt more like I was listening to a neutral and more detailed open-back Senn than an AKG frankly, but I like them a lot. They're not as "smoothed-over" as the Senn house sound and they still have the prickly edge that makes the AKGs burst into 3D life by comparison. If you get a good seal, you'll get TONS of bass out of these, and that's really nice in an AKG. You will give up detail, soundstaging, and the some of the lifelike 3D quality so abundant in the K/Q 70x though. I'll definitely be hanging on to these and they'll see plenty of use in bed at night next to the wife, in the office, and for all my bass reliant tracks - for everything else though, the K702 midrange detail is what I'll be looking for.
Edited by NewAKGGuy - 9/6/13 at 9:45am