Pros: Neutral frequency response. Massive soundstage. Excellent imaging and separation. Fast transient response (highly detailed.)
Cons: Slight resonance in the mids. Relatively poor soundstage depth leading to a "void." Relatively weak bass may not be for all.
This will be a quick review summarising the K701. My rather arbitrary rating scheme is based off price ranges with 10/10 as the best for its price, 5/10 as meh and 1/10 as horrible.
My review in one line:
Overall Sound (9/10): Very neutral. Slight resonance in mids. Detailed reserved bass. Excellent soundstaging and imaging.
Although the construction is mostly plastic, the headphones feel solid. The design is quite striking and the full size ear pads are expectedly huge. They are reasonably light, and everything from the wire frame to the velour cups feel stiff… although not very durable. Despite the solid feel, they still look quite delicate (especially the plastic suspender + elastics.) These ARE studio cans, so I doubt they would stand up to much abuse.
They are very comfortable and the cups do not touch the ears at all. The only problem is the headband, as many others have noted, the bumps tend to dig into the scalp and it gets somewhat uncomfortable. I have a huge (long actually) head and wearing it I stretch the headband to max. Personally I can leave them on for about 2-3 hours without a break.
These are reference headphones, no doubt. Don't expect a single frequency bump anywhere, because the K701's have an almost ruler flat frequency response. The excellent frequency response results in near perfect portrayal of harmonic rich instruments like vocals, strings and cymbals. I burnt mine in for about 300 hours.
Treble (9/10): Excellent, smooth, detailed, medium airyness, not sibilant or harsh. Renders cymbals with stunning reality, reflecting on the remarkably clean transient response. Grain is not heard. Missing out on a tiny bit of upper treble resulting in some air being lost. A few treble peaks found through pink noise listening.
Mids (9/10): Very smooth yet detailed. Slight resonance that results in a somewhat artificial sound. Otherwise the midrange is smooth and realistic.
Lows (7.5/10): Good extension to about 30hz. Unlike what many reviewers have said, these are not anaemic, although they certainly have less bass than the HD650/600, DT880 and slightly less than what I consider neutral. Pink noise reveals a slight deficiency of deep, bassy "weight" in the sound. Control and detail is excellent, and they can be punchy if called upon by the track. The bass quantity is still slightly below par though.
Soundstage (9/10): Very, very wide. Easily the widest in its price range. Imaging and separation is superb, again reflecting on the clean transient response. Front soundstage extension is not as good as side extension, which results in a slight "void" in the centre of the soundstage. Varies with the recording, but with an inner boundary of closeness.
Value (?/10): At $200-300, these are the cheapest out of the old flagship trio, the HD650, DT880 and K701, and yet they have a similar level of performance. These have the least bass and the biggest soundstage. They are also the fastest in responding to transients. A similar headphone would be the HD600 with more warmth and bass with slightly less detail, soundstage and speed, or the HD800 for a clear all round upgrade. For monitoring the DT880 may be better, and the HD650 has a slightly coloured, warmer more relaxing sound, but the K701 has the best balance between monitoring and recreational listening. Reading all the other K701 reviews, there's always been a lot of K701 hate going around. I can see why many people dislike the K701 because it isn't as relaxing as the HD650 nor does it have the monitoring orientated sound like the DT880. There are people who claim the K701 have a horribly plasticy sound that kills the experience. I can tell you (after listening side by side with the world class HE-500's) that the slight resonance in the mids hardly detracts from the whole experience. In fact, vocals are among the best in its price range. There are those who claim that the soundstage is unnaturally large and ruins intimate music - certainly untrue as the size of the soundstage varies - within limits - according to the recording. And then there are HD650 fans… I can personally guarantee you that most HD650 fans will hate the K701's. Their sound signatures are very polarized and as a result, it's difficult to like both at the same time. There is little doubt that the K701 perform very well technically - and better than its older siblings the K501 (the K701 is bass light, but the K501 is the definition of anaemic) and K1000 (the outdated drivers simply can't match the transient response of the K701.) Should you buy the K701? Or the HD650 or DT880? I can't tell you, but what I can tell you is, as always, to give all three a listen and then decide.
Amping, Burn-in and Genre Synergy: These headphones are deceptively hard to drive and need a massive amount of current. I could drive them with my iPod touch decently but I'd recommend the much cheaper ATH-ad900 if you don't want to buy an amp. An amp improves bass and dynamics significantly. According to measurements they draw even more power than some Orthodynamics, which is just insane. I'm using an O2 to drive mine, but anything around the $100-200 price range such as the M-Stage should be fine.
As for burn in, I used to be a disbeliever of burn in until I heard these. 150 hours will smooth the treble significantly and expand the soundstage, a rather pleasant change. Numbers as high as 300 hours have been quoted, but I found by 150 hours there was little change.
The K701 is perfect for classical/instrumental with its wide pinpoint soundstage and balanced sound sig, presenting instruments like pianos and violins in stunning clarity. Personally I find these satisfyingly bassy and aggressive (treble wise) for rock, alt rock and rockpop which is what I primarily listen to. The K701 will suit a majority of genres though.
A note on head positioning:
The way the cups are positioned on the head has an extremely noticeable and dramatic effect on soundstage. This is made worse by the fact that the cups are unusually large. As a result, improper positioning is very detrimental to soundstage, which can be exacerbated by head/ear geometries and may be the cause of the many differing opinions on the K701. Strange positioning can also affect the mids. Basically, try before you buy, because the sound actually varies a great deal depending on the person (unlike, say the HD650/IEMs/etc.)