Youtube review: http://youtu.be/GGP_y57-Bi0
Sources: iPhone5 alone, iPhone5 with FiiO E07k using LOD, various computers using the Audioengine D3 and HRT Microstreamer DACs/amps.
First impression of the KRK KNS8400: An amazingly smooth sound and perfect midrange, a slightly shy bass, and a treble that's nearly perfect (and may actually be perfect for most users). I like treble that's just on the shy side of the so-called "neutral ideal" a la the Sennheiser HD800, but if my ideal were 3 db shy of the HD800, the KNS8400 is another 3 db or so less than that, or about 6 db less than the HD800. But that evaluation of the KNS8400's treble is coupled with a bass that's approximately 3 to 6 db shy of the HD800's bass (depending on frequency), and the HD800 bass is barely adequate for many users. All of the other headphones I use now (Shure 1540, B&W P7, B&O H6, Thinksound On1, Sennheiser HD380-Pro) have a bass that's stronger than neutral, so they make quite a contrast to the KNS8400, yet I find the KNS8400 sound very enjoyable, even with my collection of pipe organ recordings.
The KNS8400 build quality seems quite good (mostly plastic except the metal headband), and although the sides of the headband project outward more than most portable headphones, the looks are decent enough for portable use. The design lends itself well to portability insofar as the earcups fold flat and they can be pulled down far enough to wear the headphone around the neck all day without getting in the way of anything. The one less-than-portable item is the ~9-foot cable, since that would have to be detached and tucked into a pocket or pouch. The headband has a fairly light clamp, the earpads have a soft squishy foam covered by quality pleather, and that combined with the large earcup interiors that don't press against any of my outer ear parts makes for a very comfortable fit.
The KNS8400 soundstage seems average for a closed headphone, and it improves noticeably with a good headphone amp. KRK specifies the KNS8400's impedance as 36 ohms, and my experience says that it's very efficient, so I would expect uniform performance on nearly any music player or headphone amp, with the only differences being the improvements that DACs and amps at various quality and price points bring to the sound. Isolation is average for a closed headphone, and leakage is such that if you're in a very quiet office in a cubicle right next to other cubicles, the adjacent co-workers may hear the sound faintly unless played at moderate volume levels. The cable is very strong and detaches from the left earcup with a twist-and-pull. The plug is the standard 3.5 mm 'miniplug' type, and comes with a screw-on 1/4 inch adapter. A very nice pleather carry bag is included, but it offers no impact protection when the headphone is packed into luggage or a backpack.
Summarizing the sound of the KNS8400, it's midrange-centric since both the bass and the treble are several decibels shy of the average frequency response of the better (high-priced) headphones sold today. Note that bass and treble strength aren't associated with quality or cost, since many low-cost headphones are very bassy, although their treble in most cases tends to be recessed rather than boosted. I noted portability above as a positive feature of the KNS8400, but portable use generally calls for a strong bass, so that may not work out for some users. As a desktop headphone at home or in a very quiet environment, the KNS8400 sounds very good to me since I can hear a satisfactory level of bass in those environments, and the sound overall has the kind of neutrality that lets the musical details be heard without being overwritten by adjacent tones.
Two of the music tracks I have that are very revealing for detail and separation of instruments etc. are Jimmy Smith's Basin Street Blues and David Chesky-Wonjung Kim's Girl From Guatemala. The former contains some massed horn blasts that resolve very well with the KNS8400, and the latter has a long burst of very intense and very bright treble sounds from a xylophone (most likely) that will congeal into a high-frequency soup with many lesser headphones. In previous reviews I've included the following music examples with comments about how the headphones sound with each track. My suggestion is instead of reading each one as an absolute unto itself, you could compare my notes here to those other reviews and see how the KNS8400 compares with each individual track.
Ana Victoria - Roxanne (Pop Vocal): Spacious sound, good bass tone, and the vocal sounds very natural. Excellent reproduction by the KNS8400.
Ben Goldberg - Root and Branch (Jazz): Realistic you-are-there sound with great instrumental reproduction. The KNS8400 plays this extremely well.
Benedictines Of Mary - O Come Emmanuel (Medieval/Female Choral/Acapella): Very spacious sound and natural reverb for a large recording venue (cathedral). The KNS8400 makes the voices come alive.
Black Sabbath - Iron Man (Classic Rock): Excellent instrumental detail - the vocal sounds very natural. As with most classic rock tracks, there is very little or no deep bass. The KNS8400 plays this music very smoothly, and the lack of deep bass doesn't unbalance the treble.
Candy Dulfer - Lily Was Here (Jazz): Narrow soundstage, but excellent detailed instrumental tone. The KNS8400 gives this a reasonable sense of space, but in spite of being a modern recording, the net effect is only slightly better than enhanced mono.
Cantus - Danny Boy (Traditional/Male Choral/Acapella): The KNS8400 plays the voices with enough low end warmth to sound very natural.
Chris Isaak - Wicked Game (Pop/Rock): The KNS8400 plays this high treble energy recording with perfection - the voice and instruments are highly detailed but very smooth.
Daft Punk - Lose Yourself to Dance (Electronic/Disco): Less than hi-fi quality recording, but the voices are very good. There's a little bass impact, but the bass doesn't have much detail.
David Hazeltine - Fur Elise (Jazz): A very high-quality recording from HDTracks. The KNS8400 reproduces the instruments perfectly with a you-are-there ambiance. The wire-brush-on-cymbal harmonics are very extended and detailed.
David Lynch-Lykke Li - I'm Waiting Here (Soundtrack/Vocal): Dark, moody song - Lykke's voice is very detailed, the bass impacts are soft, but most of the instrumentation is kept in the background. The KNS8400 plays this music very well given the sonic limitations.
Dream Theater - Take The Time (Metal): The sound quality here is limited, but the KNS8400 is smooth enough to bring out the details in this very busy music without verging on harshness.
Genesis - Follow You Follow Me (Pop/Rock): The KNS8400 plays this old and less-than-ideal recording well enough to enjoy, but the soundstage is fairly narrow.
Giant Drag - Wicked Game (Pop/Rock): Annie Hardy's version of the Chris Isaak hit has a lot of energy, but the quality is limited - still the KNS8400 pulls out enough detail to be a pleasant listen.
Grieg (Beecham-Royal Philharmonic) - Peer Gynt-Solveig's Lullaby (Classical): This very old (late 1950's) stereo recording must have been made on the most expensive gear in the world, since the overall sound quality and especially Ilse Hollweg's amazing voice are as close to "being there" as I've heard with some of the better classical recordings made since the year 2000. The KNS8400 makes this an outstanding listen.
Hubert Kah - The Picture (New Wave): This track has great bass detail and a moderate amount of weight at the same time, which I find unusual for this type of 1980's pop music. The KNS8400 plays this music very well.
Hugo Audiophile - 15-16 (Electronic): I'm not sure what the 15-16 stands for - perhaps track numbers from a CD album. The strong deep-bass tones that start around 33-34 seconds into the track reproduce lightly with the KNS8400. This is a great recording for evaluating whether a headphone's bass will be sufficient for most environments, since for many headphones that have a weaker bass, the deep bass gets absorbed and mostly lost when the environment contains a lot of low-frequency energy.
Korn - Another Brick In the Wall (Rock): Aggressive rock that's very satisfying for hard-rock fans. The KNS8400 plays this perfectly, which is to say, with proper edginess and bass impact, yet without unintended sonic harshness.
Kunika Kato - Fur Alina (Vibraphone): A very unusual instrumental - the tone quality is unlike anything I've heard before. Recording close-up is part of the magic here, but the KNS8400 does the rest in reproducing the full harmonics of this amazing instrument.
Michael Buble - Nice 'n Easy (Easy Listening/Jazz): This is the only track I bought by Michael Buble, but it's a great recording and vocal performance. The sound of the backing band here is rendered extremely well by the KNS8400, and the voice isn't pumped up for Loudness Wars thankfully.
Michael Tilson Thomas - Rhapsody In Blue (20th Century Classic): Great sound and soundstage, and terrific piano playing and tone, brought to life by the KNS8400. There are some subtle deep bass impacts starting around 38 seconds into the 17:24 length track, and the weight of those impacts can be felt with a close listen.
Muse - Madness (Rock): The bass in this track has good impact and detail with the KNS8400, and although the voice is somewhat forward, it doesn't interfere with my appreciation of the bass line here.
Phaeleh - Afterglow (feat. Soundmouse) (Electronic/Vocal): The instrumental sounds that begin this track are played very nicely by the KNS8400, but the voice tends to overwhelm those background sounds - until the heavy bass impacts kick in. If there is any doubt about whether the KNS8400 will play heavy impactful bass with good detail (if such sounds are really in the recording), this track is the proof. If you were to begin your KNS8400 listening with this track, you might think you were listening to a headphone that has an enhanced but tight and detailed bass. Simply amazing.
Richard Strauss (Mester-Pasadena) - Also Sprach Zarathustra (opening) (Classical): The granddaddy of bass is in the opening 1:50 of this recording, and I've heard it only once on a large and expensive loudspeaker system in Cleveland. For most people, that experience would be indistinguishable from being in a fairly strong earthquake. The KNS8400 conveys a little of that low-end thunder (as would be experienced on most good headphones), but most of the excitement in this track will come from the tympani, which are pretty amazing for the strong impacts they have.
Sargis Aslamazian - The Sky is Cloudy (Classical/Armenian): The National Chamber Orchestra of Armenia has a great classical program, and the KNS8400 plays this music with good separation, tone, and big-orchestra precision.
Satri-Tomoko Sonoda - All The Things You Are (Jazz): This track came from Bakoon Products, who make high-quality audio amplifiers. There's a lot of upright bass plucking in this track, and the KNS8400 plays it well, although it's recorded pretty close-up and may sound somewhat boomy at times.
Tommy Smith - Johnny Come Lately (Jazz): Small-combo jazz - sax, piano and drums. The sound is fairly close-up but well-recorded, and sounds very nice with the KNS8400, although the wire-brush-on-cymbal harmonics are not as extended as on the David Hazeltine track above.