tdockweiler already wrote a review of these, but I've been so blown away by them that I had to sing their praises, as well.  A shout-out thanks to him for pointing me in this direction. 
I was in the market for some closed cans with good isolation, clarity and durability. I was looking at the Shure SRH 940s, the Fischer FA-003s, and the Beyerdynamic DT 250-80s. In the process of looking, I was recommended the KNS series by KRK. I had heard of them, but remained hesitant, since judging by the pictures the earcups looked fairly shallow (like the Sony MDR-V6), and comfort is a very big deal for me, so I ignored them. I decided to give them a try based on the suggestion, though. I first tried the KNS-6400s, and the comfort and sound quality were okay, but the 6400s had this perceived aggression in the mids and highs, and over the long term were a bit fatiguing. After wearing them for about an hour, they started to be uncomfortable, as well. So I returned them and got the KNS-8400s, instead. What a difference! All the issues with the 6400s were resolved, and much more besides.
The 8400s are far more comfortable over the long term. I've had them on my head for over four hours today, and the headband cushion sometimes needs some adjusting, but the ear cushions are very comfortable, and my ears don't contact the insides of the earcups; or, if they do, it's not enough to bother me.
When I hear about a pair of headphones designed to be used as monitors, I have a preconceived notion of that pair being perhaps a bit thin and lifeless. The 8400s are far from it. They are very rich and full. And when I hear that description, I'm inclined to think that pair is bassy and warm, and poorly extended in the upper registers. The 8400s do not match this perception, either. It is true that they are perhaps slightly warm, and they have excellent bass impact and depth (if they had more, I'd call them bass heavy; if they had less, I'd be dissatisfied), but do not think that the highs are left out in the cold. They are very present, and I think I can say that the highs on the 8400s are perhaps the best extended out of the 40+ pairs of headphones I've tried in the last several years.  Perhaps not the best when isolated, but best when considering how well they fit in with the rest of the sound spectrum when comparing each individual model. What makes the highs so great is that they're very well extended, but not fatiguing or offensive. These headphones are extremely competent across the whole spectrum. Each part has its place. They are very clean and crisp, while remaining rich and full. In my experience, that is a rare combination, especially for models in my price range. If you had told me these cost $250, I would have not been surprised. I would gladly pay retail. They are that good. For prices seen online (~$130), they're a steal.
In my search, I believe I have found a great combination of my criteria. They are closed, they provide excellent isolation (the memory foam cushions produce a great seal), they are very clean and detailed, and they are presumably quite durable (spring steel headband, replaceable cushions and cable). I was planning on using them for a job requiring critical listening, but I'm very happy to say I can use them for enjoyable music listening, as well. If I had to criticize them, sometimes the treble can be a little hot on some songs, and the soundstage is not immense. But I've only put a few hours on them so far, and a smaller soundstage is to be expected for closed headphones. I firmly believe that everyone should give these a try. At these prices, I don't think you can afford *not* to. With their low impedance, portability, comfort, and excellent sound quality, I think they'd be a fantastic first headphone. Unfortunately, it took me a lot of money and time to find them. All in all, very highly recommended.

Thanks for the review! I always felt like the KRKs since release were a bit under-appreciated on Head-Fi. I guess it's perhaps because KRK just started making headphones.
 
If more people tried the KRK memory foam, they would NOT want to go back to any sort of velour or pleather. I think 8400 is one of the most comfortable smaller headphones ever made. It's up there with the D2000 and K601 when it comes to comfort.
 
I think soon we'll know how well they compare to the Shure SRH-940. Probably can hold up pretty well in many areas.

Heya,
 
Thanks for the thoughts, I can't wait to try out a few KRK models at this point. I'm also very interested in their ability to perhaps have velour pads swapped on, and how it would effect the sound in a negative way or not. So I'm interested to see how the memory foam holds up for me against velour, my main issue with non-velour is that I don't like sweaty headphone comes sliding around here in humid Florida when I'm outside (not an issue when I'm inside of course, thank the heavens for AC).
 
Very best,

The 8400s *are* comfortable, but IMHO they still can't touch the K601s or D2000s.  The headband is a bit annoying.  If there were a scale, I'd rate the K601s/D2000s as a 9.5-10, and the 8400s as an 8-8.5.  Still more comfortable than a lot of headphones, but not the last word.
Some more impressions:  again, despite the monitor moniker, I would describe these as meaty.  I love how well drums are reproduced.  Very satisfying.  And I was tempted to describe these as crisp, because they're very detailed and clean, but I think they're more on the smooth end of things.  Everything is there, and they're very well extended, but there aren't any sharp edges.  I might describe them as milky.  Not muffled, like a piece of fabric over the drivers, but things are perhaps a bit softer than they might be otherwise.  Just a little bit, though.

I have a pair of KRK VXT4 monitors on my desk at home and love them...... thanks for the heads-up on their headphones!

The Eagle has landed.. about an hour ago..
 
Very preliminary thoughts:
 
Love the memory foam pads.. rival my 003 in terms of comfort.
 
Out of the box, they're quite balanced sonically.. tilting to the bright side & a little edgy.  Bass isn't all that impactful but it extends deep and has good presence.. what I would expect from a studio monitor.  Mids are very much in line with the rest of the spectrum.. highly engaging due to the balance and excellent tonality.. but not forward or recessed.  The mids and lower mids have a very slight touch of (a welcome) warmth.  Treble is crisp and clear.  On the surface, it sounds like the treble is all about being crisp.. not so fast my friend!  Spend some time with them and you'll notice there's a very articulate touch of smoothness at the edges of notes.  I hear this quality across the spectrum and it's really nice.  The treble was not over emphasized and not sibilant... but like the rest of the frequencies: highly detailed and rich.  There's a very slight spike I hear in the mid/lower treble.. with certain higher pitched snare hits.. but this could recede with burn in.
 
Soundstage is average.  Despite this, nothing sounds congested or muddy.  Every instrument and sound fills the stage with balance and impressive accuracy.  Again, what I would expect from a studio monitor.
 
Clarity & detail.. right off the bat.. are as impressive as I've ever heard.  They aren't aggressive sounding phones so it not like all this is being shoved in your face, but it's very easy to hear & separate the microdetails that phones costing much more than these would miss.  This is.. flat out.. the most impressive part of the phone.  Marximus really hit the nail on the head when he said despite being labeled "studio monitors" (and the "boring, clinical" sound characterizations that inevitably would follow), these have a very rich, engaging sound to them.  I find them quite versatile if you're after detail, clarity, and a neutral (but natural) sound.  My aim to have these as an alternative 
 
Out of my iPhone 4, they sounds great.. but amping them clearly shows how dynamic these phones can sound.  I want to hear the bass gain some of the same richness the mids and treble possess.  Don't get the idea the bass isn't good.. it's quite competent.  It says more about how impressive the mids and treble are.
 
So far, I'm very happy with these phones and they look like keepers.  They work really well for music.  The detail, clarity, and tonal balance is simply astonishing.. astonishing.
 
More impressions to follow after some serious burn in.

Glad you like them.  I also found them to be *very* competent out of my 4G iPod Touch.  Possibly even better than my computer setup, although I don't think my Denon receiver is exactly awesome with its headphone out.

 
Quote:

flysweep said:

More impressions to follow after some serious burn in.

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Glad to know you like them. I'm also glad to know that you pointed out that the mids are not forward or recessed. On some reviews they claim the mids are too forward, but I never understood this. I think the mids on the 8400 are good so that you can go 8+ hours and not have them fatigue your ears.
 
On my first pair back when they were first released, the treble was extremely painful out of the box. I could have had a lemon, but it went away after burn-in. My 2nd pair sounded prefect out of the box! I never really felt the need to burn them in, but it's still a good idea. The second pair was so much smoother sounding than the first. Not sure why!
 
I can't say it enough, but I think the KRKs would be absolutely perfect for a closed budget headphone for competitive gaming.
 
The thing that really impresses me the most with the KRKs is the level of detail and how clear the sound is. I think any extra bass on these would hurt them. My pair is out on loan and I miss them already!
 

I listened to these for 4 hours straight into the wee hours last night.. took 'em off to hit the hay and didn't feel an iota of ear fatigue.  Awesome.  I did have a slight bit of listening fatigue cause of the edgy highs I experienced out of the box.
 
Now.. eight hours or so after burn in:  I'm listening to them unamped from my iPhone 4 and they sound fantastic.
 
The bass has fleshed out considerably.. much better texturing & impact.  It's still monitor like in presentation but a big improvement over how they sounded when I got them.  Surprisingly dynamic. The mid bass has filled in nicely so there more weight to notes in the midrange.. I'd like to hear a little more decay though.. the lack of that characteristic could be attributed to design & specific tuning as a studio monitor.
 
The edgy upper frequencies has begun to wear off but it still peeks it's head here and there.  Again, much less often than it did out of the box.  That smoothness I heard at the edges of notes is more prevalent now too.  A welcome revelation since these aren't as fatiguing as they were out of the box.  They remind me a lot of how the EX600 & GR07 sound.. two phones I hold in very high esteem.
 
One thing that really impresses me about this phone is its control.. it never sounds stressed on uneven when pushed loud from less-than-ideally powered devices (DAPs).. it's very impressive really.  A credit to KRK's engineers.
 
My only issue is the desire for a larger soundstage.  It will be the deciding factor on whether I decide to keep these phones.  These phones really get everything right, kudos to KRK.. the soundstage issue is purely personal and subjective, though.

 
Quote:

flysweep said:

 
My only issue is the desire for a larger soundstage.  It will be the deciding factor on whether I decide to keep these phones.  These phones really get everything right, kudos to KRK.. the soundstage issue is purely personal and subjective, though.

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I love the soundstage on these and find it perfectly good for a closed headphone. It's probably not as large as that of the SRH-940 or D2000, but it's quite good for me. I feel it's a step up from some other closed headphones. I mean even with movies and games it's quite good and perfectly acceptable for me. I usually HATE closed headphones for gaming.
 
Of course it can't compare to any open or semi-open headphone of course.
 
I would like to hear a closed headphone that has a large soundstage. Maybe the W1000X's soundstage is huge? I'd like to hear that sometime. D2000's was really good too.
 
I also listened to my DJ100 last night for 3-4 hours in bed. Almost fell asleep with them. For such a forward sounding headphone, this is surprising! Loving the M50 pads on them.
 
Too bad I have to haul around an amp with them. For those that think the 6400 has forward mids and are bothersome, the DJ100's mids may drive them crazy! I love them though. DJ100 (and 6400) just makes music so engaging. Hard to take both off my head. My K501 needs more time on my head, but DJ100 is too addicting.
 

Thanks for the great review.
 
I had the 6400, I was steared towards the 6400 instead of 8400 because the reviews had it that the bass of 8400 is recessed. While I agreed the 6400 is a good phone I would not describe it as rich and full. In fact it reminds me of Ultrasone somewhat, transparent and detailed but lacking in tone. For the price no complaints however. I got the impression that 8400 could be an improvement  across the board except the bass area. Like to try out the 8400 someday.
 
 

I think as a studio monitor, I don't think KRK would want this. Maybe somewhat, but to a point. I don't think the 6400 is too thin sounding though and still quite musical to my ears.
I think people expecting the tonality of say an HD-600, would be majorly disappointed To me, this is a good thing and  I liked the HD-600..somewhat.
 
For those that want a warmer and perhaps(?) fuller sounding headphone, there is also the Shure SRH-840. For a studio monitor, I'd take any of the KRKs over that any day. I'm a fan of the SRH-840 though.
 
Quote:

greeni said:

Thanks for the great review.
 
I had the 6400, I was steared towards the 6400 instead of 8400 because the reviews had it that the bass of 8400 is recessed. While I agreed the 6400 is a good phone I would not describe it as rich and full. In fact it reminds me of Ultrasone somewhat, transparent and detailed but lacking in tone. For the price no complaints however. I got the impression that 8400 could be an improvement  across the board except the bass area. Like to try out the 8400 someday.

 
 

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Compared the 6400 and 8400 at a shop today. The 8400, in my opinion, are the clearly better headphone, disregarding voicing preferences, tonal balance, FR and the such. Listened to some Ian Carr's Nucleus, and the 8400 made everything more clear, were it bass articulation, pitch accuracy across the spectrum (listened to some guitar action meddled with saxophone, trumpet and oboe) or discerning drumming techniques. About as "airy", but overall musicality is so much better with the 8400s. Bass drum had it's rhythmical element and was separated from electric bass very well. You could also hear the drummer play with the cymbals and hear him hit it at different spots, creating different pitches and volume. I think the word would be "dynamic treble", hehe. Not overly done at all. Nor were they bright to me, though a bit more apparent than with the K272's, but they are a tad smooth. The dynamics and detail were better with the KRK nevertheless. This wasn't due to coloration or illusion of more detail either. The drummer just sounded more natural with the drums and I heard musical cues that weren't audible with the K272. Nice
 
While the 6400's completely lost to my K272HD's, the 8400's did clearly improve on them in many areas, at least based on that half 'n' hour listen. That's why I put them on order straight away, so I can listen some more. 
 
Not to completely shoot down tdockweiler's impressions and opinions, but the 3-4% improvement is an understatement for me. I don't listen to vocals much, but have noticed that they don't tell much if at all on headphone's performance objectively. That's more of a personal thing and preference issue and I have nothing against that. I've come to understand you listen to female vocals quite much and have made much of the impressions based on that. Please correct me if I'm wrong.
To me the improvement, however, if put into numbers, would represent something like 30-40%, since the 6400 were pretty monotonic in comparison. 

Quote:

electropop said:

Compared the 6400 and 8400 at a shop today. The 8400, in my opinion, are the clearly better headphone, disregarding voicing preferences, tonal balance, FR and the such. Listened to some Ian Carr's Nucleus, and the 8400 made everything more clear, were it bass articulation, pitch accuracy across the spectrum (listened to some guitar action meddled with saxophone, trumpet and oboe) or discerning drumming techniques. About as "airy", but overall musicality is so much better with the 8400s. Bass drum had it's rhythmical element and was separated from electric bass very well. You could also hear the drummer play with the cymbals and hear him hit it at different spots, creating different pitches and volume. I think the word would be "dynamic treble", hehe. Not overly done at all. Nor were they bright to me, though a bit more apparent than with the K272's, but they are a tad smooth. The dynamics and detail were better with the KRK nevertheless. This wasn't due to coloration or illusion of more detail either. The drummer just sounded more natural with the drums and I heard musical cues that weren't audible with the K272. Nice
 
While the 6400's completely lost to my K272HD's, the 8400's did clearly improve on them in many areas, at least based on that half 'n' hour listen. That's why I put them on order straight away, so I can listen some more. 
 
Not to completely shoot down tdockweiler's impressions and opinions, but the 3-4% improvement is an understatement for me. I don't listen to vocals much, but have noticed that they don't tell much if at all on headphone's performance objectively. That's more of a personal thing and preference issue and I have nothing against that. I've come to understand you listen to female vocals quite much and have made much of the impressions based on that. Please correct me if I'm wrong.
To me the improvement, however, if put into numbers, would represent something like 30-40%, since the 6400 were pretty monotonic in comparison. 

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See..this why I love Head-fi..all of our impressions are very different. Sometimes, but not always. I do feel it's best to give both a long, long try at home to REALLY nail down their differences. Sometimes it's easy for a headphone to "Wow" me at first, but then after month one I actually find out I don't like it or find out more of it's negatives! The 6400 has been with me for a few months now without complaint. As I mentioned in my review of both, it's best to pick one based on your musical preferences. Everyone's idea of what's "musical" for them is totally different. Strangely enough, for my music the 6400 is more "musical", fun and engaging. It really got me into my music so much more than the 8400 did. I love the more forward mids of the 6400 and prefer the 6400 for vocals by far. For some, the 8400 will easily be better for them. For example, if I listened to classical, there is no doubt I'd be picking the 8400! For me, the 6400 is closer to my favorite headphones than the 8400 was.
 
I still think that as a studio monitor the 8400 might win with most people. The one reason I prefer the 6400 is that it was just more fun to me to listen to. This is why I prefer the K501/K601 over the technically better K702
 
Now, I'm sure someone with better trained ears can pick apart the differences between the two much better than I can (like you). I had both at the same time for about 3 weeks and found it difficult. On SOME music it's super easy and on others it's impossible! It varies between songs. I listened to most every type of music and I still believe the differences were not like night and day to me with most music. Some will probably think otherwise! Sure 3-4% is maybe too low, but I don't want to make people think the differences are massive. I just wasn't hearing it. Now there is certainly a chance there are variations between pairs. It looks like the 8400 has maybe a small mid-bass hump (I didn't hear it on my pair?!), so this can totally effect it's tonality and impression of being more musical for some..I don't know. The easiest way to determine the differences was in anything with a large soundstage. Some acoustic music did the trick. I did feel the 8400 had a better soundstage, but this seems impossible and could be my brain playing tricks.
 
As you've done, it's just best to try both and pick the winner! Some will like the 6400 and others will prefer the 8400. It seems I'm the rare one who prefers the 6400! I only listen for fun and don't run a studio though. I do admit that I listen to a LOT of female vocals and the 8400 was just not doing it for me in this area. That is probably a big reason I went with the 6400. So far I've heard of about two that have preferred the 8400 over the 6400!
 
To me, it's like asking people to pick between the K501 and K702, but the differences are even minor.
 
Also..forget to mention, my impressions are based on all different genres. It is true that a lot of my favorites focus on vocals, but it would have not been fair to judge the 8400 just based on this.
 
When you've heard the 8400 even more, I think we'd all like to more impressions you have. Can't say it enough, but I think both headphones are quite good for their asking prices!

 
 

Of course you are right about the subjectivity. (Kind of eats away from this site though ) I might find negatives later on, but it was quite apparent to me by my standards, which was the better one. I'm more confident I'll get used to the minor differences to my current reference, the K272. 
 
3-4% might be about right for tonal issues. I wouldn't be able to comment on that, my hearing is not that good... Or I just listen to different things
 
The 8400, as I said, was the clear winner. Let me quote myself: "disregarding voicing preferences, tonal balance, FR and the such." Objectively, they simply were better at distinguishing notes, especially the brass and bass I got to hear from the Nucleus albums. With the 6400, I could not hear the bass groove nearly as well as with either my K272 or the 8400. The tonality was good and I had no problems with that. But it was weird how much they differed in the aforementioned aspect... For a musician who seeks to be able to tab notes, for instance, the 8400 just win hands down. I don't think I need to spend any more time with the 6400, since I don't care about sound itself, but the musical accuracy a headphone delivers. By this I simply mean: to distinguish variations in pitch. I've noticed that if a headphone does this right, it excels in many other parts as well. 
 
I didn't pay attention to the forwardness or soundstaging of the mids, but the 8400 were cleaner and more clear, were it indeed guitar, drums, trumpet, saxophone or oboe. For simpler music, without as much melodic layers, tracks, harmonics and modality, the 6400 might do just as good and I'd be much more hard pressed to tell the difference. Put on some Zappa, Miles Davis or indeed Ian Carr's Nucleus (more groovy) and the difference in performance speaks for itself. 
 
Anyways, can't wait to get them. They were highly comfortable as well. Very light, felt kind of cheap and hollow with the plastic, but not too breakable. Expected more from the memory foam, but it did the job and was at least better from the 6400 pads.