Its been long enough that I feel comfortable in my knowledge of the K712's abilities enough to post a summary/review. I'll mainly be comparing them to the DT880 and HD600.
The K712 is a great headphone with a lot of strengths and a couple of weaknesses. How you feel about its weaknesses will determine how much you like the K712 IMO.
The soundstage is very wide. Its pretty impressive and I really like it. However, it is no deeper than comparable open cans like the DT880 and HD600 which is unfortunate. The DT880 and HD600 (and similar open cans) already have decently wide soundstages so widening that without making it deeper kind of sounds slightly strange. I get the feeling that the depth should be more, in order to match the width, and that that would result in a pretty amazeballs soundstage that I wish I could experience.
The sound signature is fairly smooth with good extension at both ends. The bass is of quite good quality and is quite satisfying but it doesn't entirely escape the "plasticy" issue (more on that later). It is not over emphasized but not lacking and has a pretty good amount of very low end extension for an open headphone. For me the bass quantity is just about perfect.
Detail is pretty great on these. I get the impression that they have ever so slightly more detail than the DT880 and HD600 but its hard to tell if its "real" detail or just an effect of the emphasized upper mids. Every little thing is revealed in full light in that range. There's also a decent amount of treble (its not warm or rolled off in the treble to my ears) and there's a good amount of detail there as well. The bass is also pretty well textured with a good and satisfying amount of detail.
The comfort is awesome. However, I found myself being slightly bothered by the extremely low clamping force. I'm only ever bothered with it after medium to long sessions where they can feel too loose if I'm moving my head around at all. But its a fairly small gripe and they are definitely very comfortable and the memory foam velour pads are awesome.
Personally, I think they look cool. There's a nice matte finish on the orange accents that makes it look much less bright and gawdy than some of the pictures make it look. I really like the design and the color scheme, I think they did a good job on it. Any more orange and I probably wouldn't like it as much but they struck a good balance.
The fact that it comes with both a long straight cable and a shorter coiled cable, and that they are detachable, is definitely worth mentioning IMO. I wish this was a standard for all mid- and hi-end phones.
1- Tonal accuracy
The boosted upper mids of the K712 doesn't do the tonal accuracy any favors. They have a "plasticy" sound to them. Tonally the DT880 and HD600 are both clearly more accurate though its not a huge difference.
The boosted upper mids again cause the second problem-- fatigue. In terms of fatigue, the K712 is more fatiguing than the DT880 and much more fatiguing than the HD600.
For me, fatigue level and tonal accuracy are pretty high up on my list. As such, overall, I consider the K712 inferior to both the DT880 and HD600 and I use them less than I do those cans. Based on the asking price of the K712 when they first came out you would assume they are an obvious step up from headphones like the DT880 and HD600 but they aren't. They are very good and are worth the price IMO, and many will prefer them to the DT880 or HD600 and/or consider them technically superior to those cans in one way or another, but I do not think its accurate to say the K712 is in a different league from the DT880 and HD600. They are most certainly in the same league and it will come down to preferences.
That's all I can think of at the moment. I'm happy to answer any questions. Overall I do like the K712, quite a bit, but I prefer the DT880 and HD600 and use them more often than I do the K712.
I did hear some oddity in the upper mids and I did find them a bit fatiguing when I listened to them. After listening to them, it eliminated any desire for me to get them as an upgrade from my K612 as I found the K612 more tonally accurate and less fatiguing than the K712, I just wish the K612 had a detachable cable. I do want to try the K712 on more systems and see if it was simply a case of synergy that isn't ideal before coming to a concrete decision about them.
Edited by kman1211 - 12/21/13 at 11:07pm