I've had my K7XX for over two years now (January 2015 drop), SN#4329 and these have never let me down! I use them mostly for music but also for gaming and drive them with an Audio-gd NFB-11. After a month long, intensive comparison with the K712 I owned before these, I sold the K712 and the K7XX have been my primary headphones ever since.
Power requirements: I've had several desktop dac/amps over the years and sold a Creek (Music Hall) OBH-11 amp two years ago when it was time to whittle down my collection, about the same time I sold my K712. These are NOT the most efficient headphones on the market! They do need a pretty hefty amp section to drive them over ear damaging levels. That said, I've plugged these into everything from my Galaxy S5 phone, Kindle Fire HD tablet, motherboard sound out, dedicated SB sound card, Dell laptop, Yamaha home theater receiver, and some generic Korean ipod clone I bought at a garage sale once... all of them, *everything I plugged them into*, were perfectly capable of driving these to usable volume levels, although maxing out the volume on the weaker devices may introduce audible distortion as is the case with any headphone, earbud or speaker. It all depends on how loud YOU need to listen...
Do they enjoy more power? Yes. Do you *need* an expensive dac/amp to use these? No.
Build quality: Flawless. Still bend and twist the same way they did new out of the box, never broken anything on them after hundreds of hours of use.
Comfort: Like the K712 I had previously, these are the most comfortable headphones I've ever worn, especially for the long haul during several hours of online gaming. Light weight, extremely soft ear pads, none of the head band bumps that made some of the earlier 700 series infamous (I had a pair of the original fixed cable K701 for years... the headphones that I loved to listen to but hated to wear for more than minutes at a time!)
Sound quality: I'm admittedly an open backed, AKG fan boy. The K7XX, and the K712 which I personally found to be identical in sound signature, just do what I want *right*. Sometimes I still think the K701 had better highs, more sparkle, but it could be that the +3db in the bottom end of the K7XX just makes me perceive these as more balanced, more "laid back". I've tried most of the usual suspects that compete with AKG and they all left me wanting, especially at often elevated prices.
These are still "spooky" good at times, sometimes throwing sounds deep in the sound field that have tricked me more than once into thinking the sound actually came from somewhere else in the room. FPS's like ARMA 3 are incredibly immersive while wearing these. Forget "gaming headsets" that advertise multiple drivers for "surround sound"... you don't need them!
My tastes in music are highly eclectic, running the gambit from large, classical orchestrations to hip hop, metal, electronica (Sphongle sounds like AKG are fans!), classic rock, blue grass and jazz. I'm particularly fond of acoustic, upright bass and cello and these do not disappoint. I won't tolerate a speaker (or headphone) that can't realistically play back a well-mic'd drum kit and here too, the K7XX are spot on from the thump of a kick and the snap of a snare to the shimmer of a brushed cymbal. Gene Krupa would have approved.
Vocals are a mixed bag but that's always the case, with any transducer. It's not the headphone that might find some vocal tracks lacking in some way but rather the recording and production of any given track. In a nutshell, the K7XX are highly detailed in their almost analytical way of not hiding anything. A garbage track will sound like garbage, a great track will sound fabulous.
Tips: The headband will deform slightly if hung for a long time on a hook. Doesn't hurt them or their comfort in any way but if you have an OCD about the appearance of such things, just before you use them, store them either laying flat or get a more supportive, curved, headphone stand.
The ear pads are magnets for dust and particularly cat and dog hair! If it's floating around in the air, these will catch it. I keep them covered with a cloth when not in use. Still I find myself using a bit of brown, paper masking tape to clean them up a few times a year. Be CAREFUL with these ear pads! Full compass charges so much to replace them (when they have them in stock) that you might be better off just buying another pair of headphones, IF you mess these up!
One thing I liked about the more expensive K712 is that they actually included an optional cord! That said, a great replacement cord is readily available from the usual sources and cheap too! I have a 1.5m one that I bought two years ago as well... it's straight, with a woven fabric outer shell that has sort of a red tint in the right light, and metal tip housings with spring relief... I think it cost me $15. Google is your friend. ;)