NOTE: PLEASE READ!! This is an update to the review posted below.
 
I owned the KRK KNS-8400 for over 6 months. Eventually I found myself not needing an extra closed portable headphone so I sold it. I always liked it, so I decided to give it another try in July 2011. I should have kept it!
The pair I received in had NONE of that painful treble. It was a lot smoother and did NOT even remotely feel like icicles stabbing me in the ears. The bass was also quite good, but MY pair still lacked in bass impact. The mids sounded the same, but it's signature felt a tad more natural. I have absolutely no clue why or how, but the soundstage felt a lot better. I don't think I had a lemon, but most likely was just some strange variation between pairs. Mine was a very early pair. Overall I prefer the KNS-6400 for my music preferences.
 
You can find a review and some comparisons to the 8400 here:
http://www.head-fi.org/forum/thread/560605/krk-kns-6400-review-impressive-99-giant-killer
http://www.head-fi.org/forum/thread/540079/five-headphones-compared-kns-8400-dj100-hd-598-sextett-and-mdr-7506
 
 
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The other day I saw these in stock at my local Guitar Center and I needed some new headphones that might sound like the Shure SRH-840 but were more comfortable, cheaper and much smaller. These looked like they might fit that description. I have my favorite DJ100, but that requires an amp at all times. I wanted an un-amped portable headphone that was not very large, but still sounded like a $150-$200 headphone.

The comfort on these is quite impressive. The most comfortable headphones ever to me is a tie between the k240 Studio and the Denon D2000. The k240 Studio might be the winner due to zero clamping force and how light it is. I love the D2000 pads though, but the headphone isn't that light. I also find the Maxell DHP-II extremely comfortable.

COMFORT, ISOLATION, BUILD QUALITY

These headphones have two bumps on the headband that are memory foam. You press on them and it takes awhile to come back to it's original shape. You cannot feel the headband against your head at all. The earpads are memory foam I guess, but they feel like the ones on my old Bose Triports (don't ask!) and my DHP-II. The comfort overall on these headphones is impressive. M50, SRH-840, DJ100, HFI-680 and similar headphones can't even come close. On my small head there is very little clamping force. Isolation is OK. Good, but probably not great. Comfort is a 9/10. My D2000 might have gotten a 10/10.

The build quality of the headphone is fine. I do have major issues with the cable. It feels and looks ultra cheap. It's as if they skimped on quality here. For a $150 headphone that's not acceptable. Have you ever gotten a headphone and the cable has that cheap rubbery feel? Think of the cable on the Creative Aurvana Live or k420 and it's just like those (but not as thin of course!). After a day it still retains part of the shape it was in when it was in the package. Part of it looks like it wants to stay in a square shape! It also seems to want to tangle. When I have the headphone on my head and I move the cable I can hear it a bit. Not when I play anything of course. The cable is removable and the volume control is a bonus and DOES NOT need to be attached!

The headphone is actually quite small. One of the smaller full sized headphones at least. They make the M50 and SRH-840 look massive in comparison. These would make a perfectly good portable headphone that sounds good straight out of an Ipod! They don't fold  up, but the cups can lay flat. On my Ipod Touch 2G un-amped I need to put the volume 4 notches from the right to get enough volume. With my k240 Studio it has to be maxed.

SOUND QUALITY

I'm a huge believer of burn-in. I wasn't at first until I noticed some major differences with my k240 Studio, DJ100 and k601 after burn-in. I've found that many headphones really don't need burn-in, but some do. I usually like to listen to them on the first day and then burn them in when I'm sleeping.

This is a real Jekyll and Hyde headphone. My first day of listening to them was not a fun experience in any way (it gets better, trust me). The first thing I noticed is that the headphones were VERY fatiguing. They were excessively bright and I'm talking about like icicles stabbing me in the ears bright. This seemed to not always show up in some of my favorite music, but in other music it did.

At first I was thinking something like "maybe these songs are supposed to sound this way", but that's definitely not the case. I figured maybe the headphones would need to be amped and I used my Asgard. At first I thought it was a bit better, but not really after all. I didn't notice any differences between amped and un-amped other than volume increase. I didn't even have the volume that loud and when I took a break my ears were just ringing so badly.

Despite the piercing treble there was enough to like. The mids were very good. Vocals were crystal clear and easy to hear. Detail was almost at the point of being excessive. I think these are possibly one of the most revealing headphones I own. Imaging is very good and reminds me of how it was on the SRH-440. Soundstage is good for a closed headphone. I doubt a closed headphone could ever impress me with it's soundstage. The D2000 did almost. Out of the box this headphones signature reminds me of almost like the Shure SRH-440! A little brighter and less bass.

So far the weakest area of the KNS-8400 is it's bass. Even for a studio monitor/neutral headphone it doesn't quite have enough. I'm one who finds the amount of bass on the SRH-440 and SRH-840 perfectly acceptable. On this headphone there is maybe a tad less bass than on the SRH-440. The bass seems to have improved a little with burn-in, but I honestly don't know if it's my imagination...yet. As it is right now I'm OK with it's bass, but I just would have hoped for a little more. For comparison it does have way more than say the AD700.

At one point on day one I actually thought several different things. First it was that I wasted $150 on a sound signature I could possibly never get used to. Then at one point I said "I hate these things!".  Another time I actually was thinking to myself that there was no way these were neutral and couldn't possibly have a balanced sound due to it's excessive treble. I was going to write a few impressions on day one, but that would have been a mistake and totally unfair. On day one the sound signature just felt a little too aggressive and did not sound natural in any way.

So on day two and with a little more burn-in they have improved by a LOT. The treble is now less of an issue and it's way less fatiguing in every way. No more of that piercing treble. A few songs give me issues, but they're like that on many of my other headphones, but not as bad as with day one on these headphones. On day two these are now much less fatiguing than say a Grado SR-80. Metal may be slightly fatiguing still with these headphones, but I don't listen to any of that. Fans of the Grado 325i and some Beyer Dynamic headphones would have probably had no issues on day one I bet! I may be really sensitive to treble and prefer slightly rolled off highs, but not too much. For me the treble of the DJ100 is PERFECT.

I've found these headphones to sound great for classical music for a closed headphone. It makes me want to listen to more classical music. Imogen Heap and non bass-heavy electronic music sounds great on these. For music that needs a lot of bass to sound good, this headphone is definitely a poor match unless it improves by a huge amount after more burn-in, but that's highly unlikely.

Right now on day two the sound quality seems "just right" and very balanced. Not too much of any one thing finally and just what I look for in a headphone. I've heard some of my favorite songs probably hundreds of hundreds of times and they sound no different on this headphone. The sound is very, very clear. Even possibly more so than my DJ100. Kind of like my old SRH-840.

For anyone that cares, these are way brighter than the SRH-840 for sure, but the SRH-840 has way more bass and it felt like the 840 had slightly forward mids. If I could find an SRH-840 with this design and with this level of comfort I'd be set.

I still feel it's a bit early to give them a final score for sound. I don't know for sure yet if the sound is better than the SRH-840 yet, but probably not. These are definitely going to be far better for some genres. In the overall view, these are better only due to better design, comfort and fit. The SRH-840 and the KNS-8400 would be a good comparison!

If you want a headphone to try and "Wow" you with it's sound, don't get these. They won't try and do that and that's what I look for in a headphone. Despite this they're not a boring headphone in any way and it's fun to listen to them. Basically I feel as if these won't try to change my music and make it sound better than it really is.

I can't say it enough, but this is another headphone that completely improves with burn-in. Even just a little helps. In total they've probably only gotten 12 hours of use. Tonight I've listened to them for three hours and my ears are not ringing and they're still comfortable. They have a totally non-fatiguing sound signature now. After day one this just made my day! I now really regret even listening to them before burn-in.

These cost $150 and overall they are keepers and worth my $150.  The cable is just not good enough for a $150 headphone. KRK should seriously think of including a slightly better one. I'm so glad it's not coiled though! They could just skip the volume control and include a better cable instead.

I know some people probably think these headphones are ugly, but not me. I think they look pretty good. It's a good design, but they totally screwed up on the cable.

KRK KNS-8400

sound: 8.75 (not quite sure on this yet, sorry)
comfort: 9
value: 8 (I'd add a .5 if the cable was better)

For comparison, here is how I rated my last few headphones (scale of 1 to 10):

Audio Technica ATH-M50 (black box version, white one sounds different)

sound: 8
comfort: 8
value: 9

Ultrasone HFI-680

sound: 7.25
comfort: 6.5
value: 6/10

Shure SRH-840

sound: 8.5
comfort: 4 (too heavy and impossible to balance on small heads!)
value: 6 (if you can get them at $130 change that to a 9. Not worth $200 in ANY way. $160 max)

Koss Pro DJ 100 (amp required, should leave this off the list)

sound: 9
comfort: 7.5 (8.5 with M30 pads)
value: 10 (best $80 headphone ever)
 

Thanks for sharing mate....

Thanks for the review!  I'll have to drop in to a Guitar Center and see if I can check these out.

I've been listening to them still a lot and I really like them. The only problem is that they're still the brightest headphone I own for sure. I still find the sound non-fatiguing IF I don't listen to them too loudly or it's not specific pop music (the Japanese singer "Hitomi" is really harsh on my ears at times) or metal. Only a few artists tire my ears out and I think that's how the CDs are mastered. The bass after burn-in seems to have gotten a bit better too, but it's still disappointing to say the least. My k601's treble feels really laid back in comparison and that headphone is really not like that. Of course the treble isn't like say the Grado 325i or some of of the Beyer Dynamic headphones but close. The KNS-8400 makes my well burned in k601 sound like a bass monster.
 
Still love the comfort. I had them on again for another 3 hours today (I listen to music while I have NBA league pass on TV muted!). I did burn them in another 15 hours or so.
 
I liked these so much (despite the bass) that I also re-ordered my old Shure SRH-840 (that I loved) to compare them. I haven't found a headphone that's better (DJ100 probably is, but I'm not sure yet) for under $200 yet, so now I can compare them all. I missed my old SRH-840 so much that I just had to buy another pair. I'll just hope that they magically fit better, but I doubt it. I hate how the SRH-840 is nearly impossible to find for under $150 NEW outside of ebay. Luckily "Adorama Camera" has them for $150, which is the max I'd ever pay for them. Maybe after having tried so many other headphones since I've had the 840 last, I'll like them a lot less. I hope not. I think I was forced into selling the SRH-840 because the M50 fit better. I ended up selling that and going with the k240 and now I'm back to square one kind of. I'll compare the KNS-8400 to the SRH-840 and keep the SRH-840 if it's just as good if not better. KNS-8400 I'm keeping no matter what. I'm willing to deal with it's bright signature because of it's comfort.
 
It's funny now that I have the KNS-8400 my AKG k240 is sounding pretty bad. But it's still good and worth every penny for $100. The sound is just so much clear on the 8400. That's the biggest thing I noticed.
 

I got a chance to demo the KNS-8400 today at Guitar Center, along with the Shure SRH-440 and 840.  I'd have to agree with your impressions so far.
 
The KNS-8400 is similar to the SRH-440 in tonality, except that the KRK is brighter.  Also, I think the KNS-8400 was much more detailed, enough so as to put them in another class above the SRH-440.
 
The KNS-8400 versus the SRH-840 is a more interesting comparison.  The SRH-840 is definitely the more sonically balanced of the two, offering nice, tight bass in a good but not overbearing quantity, great mids, and smooth but well-extended highs.  By comparison, the KNS-8400 is bright and light on bass.  While both have exceptional detail, I would probably give the edge to the KNS-8400, though not by a lot.  And when it comes to comfort, there really isn't a comparison -- the KNS-8400 was much better for me than the SRH-840.  The lighter weight of the KRK 'phones, combined with the two-pad headband cushion configuration and the softer ear cushions, made them much more comfortable than the SRH-840.  I didn't think the SRH-840 was immediately uncomfortable, but I got the impression that they would become uncomfortable with longer listening sessions.
 
If I were to pick on sound alone, I would go with the SRH-840.  But if comfort is particularly important, it makes the decision more difficult -- maybe a slight edge to the KNS-8400 overall in that situation.

I agree with you on the KNS-8400 being in another class above the SRH-440. I liked the 440 when I heard it, but not enough to keep them (had too many other headphones at the time).
 
I think these KRK headphones need a TON of burn-in. I gave them another 12-15 hours and they seem to keep getting better. It seems like their sound signature is less fatiguing on ANY music the more I burn them in. Not a night and day difference, but it's definitely helping. Today I listened to them for another 2 hours and liked every minute of it and it didn't tire out my ears at all again. Not once did any of my music seem harsh to my ears, which is a really good thing.
 
Hopefully I get my SRH-840 in by the end of the week. I got them on sale after waiting forever for them to show up at a good price. I originally got them for $130 and wish I would have kept them when I got them at that price.
 
I doubt the bass will get better on the KNS-8400 with more burn-in, but I sure hope so. I think that's expecting too much.
 
I'm still constantly annoyed by the cable. I wrap it up and put it in my pocket and it always stays tangled.
 
So far I'm pretty happy with this purchase. The amazing comfort makes it even better. If the SRH-840 sounds only slighter better, I'd be willing to downgrade the sound quality a little and only use the KNS-8400.
I imagine the KNS-8400 would be far, far better for classical though, but who knows.
 

Well, I bit the bullet and picked up a pair of the KNS-8400 headphones.  The more I listen to them, the more I like them.
 
While the bass is still light, it is definitely well-extended -- I was listening to the Shiny Toy Guns version of "Major Tom", and at one point in the song there is a synthesized bass sweep, starting at maybe 50-60 Hz and then going down very low, definitely into the 20s range and maybe even continuing into the subsonic frequencies.  The KRKs reproduced that sweep down to the bitter end!
 
In the end, I just liked how clean and detailed they sound.  I do find them a bit fatiguing still -- I'll probably set them up with some tunes and let them burn in for 10-20 hours or so and see if I get the same kind of burn-in changes tdockweiler described.
 
The bottom line at this point:  people who are very sensitive to treble or bassheads need not apply.  However, if you're looking for a very clean-sounding, very detailed, and very comfortable pair of closed headphones that can be easily driven from a portable DAP, the KNS-8400 is a solid option.
 
Oh, one more note:  I actually prefer the KNS-8400 from a class AB amp like my Yamaha RX-497, rather than a class A amp like my Schiit Asgard.  In my experience, class A amps sound very clean, dry, and analytical, whereas class AB amps are a little more euphonic though less detailed.  The KNS-8400 is already so clean and detailed that I think driving it from a class A amp is borderline overkill.

This thread has been greatly informative and I registered primarily to thank you all. I hate mixing in headphones, but since my studio is a small home one it is either that or drive my husband nuts as I tweak the same bar for an hour. I heard the promotional reviews for these KNS-8400s and was intrigued, but I am pretty sure I am sold after reading real reviews from you guys here. 
 
With regards to the lack of bass I am wondering about your interfaces, as none of you mention that. Many, even Pro-Audio, interfaces only provide 20-20 response through the headphone out. What interfaces/sound cards are you guys using? 
 
Thanks again, this forum seems like a great resource!

I usually use my KRK KNS-8400 with just an Ipod Touch 2G. I don't use them for anything serious like as a studio monitor or mixing. I also use them a lot with my Schiit Asgard amp or a Total Airhead. I haven't noticed any changes between them when amped or not amped. I do have a DAC I sometimes use for my PC, but lately my music is through an Ipod or a CD player. Most of my source files are Apple Lossless or 320kbps at the mininum. Ever since getting my DJ100 I've ripped everything to ALAC. I once used WAV, but that's not really a good idea for a 32gb Ipod Touch
 
The light bass isn't a huge deal for me, but it kind of takes away from the value or these headphones. They're still good enough for me to use as my primary portable unamped headphone. I've found them even more detail than my SRH-840 that I also bought a few days ago. I will say that some of the other light bass headphones have ruined the experience of listening to music on them for some reason, but these don't. Still good enough to enjoy.
 
These also are far more revealing of my source than my SRH-840. I wasn't expecting that. They're still one of the brightest headphones I've owned since the 325i. It's only with specific music of course. My DT-990 wasn't even nearly as bright! No joke! After burn in the highs become much more fatiguing. Only two artists give me issues, so that's good enough for me.
 
One idea is to get the KRK KNS-8400 and the Shure SRH-840 and return the loser If you don't need a lot of bass, the KRK may very well come out as the winner in many areas. I haven't compared them side by side yet, but may do this soon.
Quote:

byteblock said:

This thread has been greatly informative and I registered primarily to thank you all. I hate mixing in headphones, but since my studio is a small home one it is either that or drive my husband nuts as I tweak the same bar for an hour. I heard the promotional reviews for these KNS-8400s and was intrigued, but I am pretty sure I am sold after reading real reviews from you guys here. 
 
With regards to the lack of bass I am wondering about your interfaces, as none of you mention that. Many, even Pro-Audio, interfaces only provide 20-20 response through the headphone out. What interfaces/sound cards are you guys using? 
 
Thanks again, this forum seems like a great resource!

Click to expand...

Welcome to Head-Fi Byteblock, and as is our customary greeting here, sorry about your wallet! 
 
I use my KNS-8400 with a few different setups.  The KRKs are very easy to drive, so they are good for portable use (this is really the reason I got them), and I use them with my iPod Nano for that purpose.  My computer rig consists of a Bel Canto DAC3 (fed lossless audio via coax SPDIF) feeding either a Schiit Asgard or a Yamaha RX-497 stereo speaker amplifier.  I have a couple of tube amps too, but they're both OTL designs and generally not the greatest match for low-impedance headphones like the KNS-8400, so I don't bother using those tube amps with the KRKs.
 
As for the bandwidth of my setups, I'm really not sure.  I have my doubts about the iPod, but the DAC3 / Asgard should have pretty high bandwidth.  The DAC3 is spec'd as 20 Hz - 20 kHz +/- 0.5 dB, but I wouldn't be surprised if it could deliver lower frequencies than that.  The Asgard has a very high bandwidth, reportedly from 2 Hz to 200 kHz +/- 3 dB.  In any case, this setup should have no problem delivering bass down to the lowest limits of human hearing.
 
I find the KNS-8400 to have very good bass extension, reaching low frequencies quite easily.  However, the bass just doesn't have very much quantity compared to the mids or the treble.
 
Like tdockweiler said, I don't think that the KNS-8400 really benefits from having solid amplification -- however, it is revealing enough to illuminate the differences between the sources and amps that I use it with.  I'd highly recommend having a good DAC to use with the KNS-8400, as these headphones WILL let you know if there's any noise or flaws in your source.
 
Do you have a Guitar Center nearby?  If so, you can go there and demo some studio-type headphones.  I was able to demo the KNS-8400 and the SRH-840 there.  Being a rather bright headphone, I'd really have to recommend demo'ing the KNS-8400 before buying -- I like these headphones a lot, but the treble could be overwhelming for some.

I'm glad this thread is still alive. I've been listening to them for hours tonight again and have been comparing them non-stop to my DJ100 and SRH-840.
It's funny because the SRH-840 seems REALLY laid back in comparison to the KNS-8400 and DJ100! I never really felt the SRH-840 was like this until I heard these two headphones. I still like the SRH-840, but often it feels like they're designed to be easier to listen to all day. The sound just isn't as engaging as I remember, but I still like the SRH-840.
 
This sounds like a huge negative and worse than it really is, but I imagine the KNS-8400's mids would sound like those of the 840 if I put a thin cloth over it's driver
It's weird taking off the 840 and putting on the KNS-8400. Feels like my head is clamped between two pillows. Isolation is not as good as the SRH-840 though.
 
I wonder why on earth the SRH-840 is so huge. Such a huge design is totally not needed and overkill. It's like "hey, I'm huge! I must sound great!"
 
I got the Shure SRH-840 to test against the KRK. The SRH-840 just may go back. The only thing I'd miss is the isolation and it's bass. Since the 8400 has more detail and everything is more clear the SRH-840 isn't much better.
 
Right now I think my Koss Pro DJ 100 and KRK KNS-8400 sound better than the 840 in nearly every category (except for the bass with the 8400). Pro DJ 100 and KNS-8400 have about the exact same amount of detail and the sound is just on clear on each of them. The DJ100 though is less comfortable, absolutely must have an amp, but has a LOT more bass and less fatiguing highs. It's crazy that I'm even comparing an $80 headphone to $150-$200 headphones. DJ100 and KNS-8400 are now probably my two favorites under $200. SRH-840 still is, but now I've finally heard and found better headphones.
 
I think people who like the KNS-8400 would also like the DJ100.
 
I know this isn't everyones thing, but I think the KNS-8400 would make a good gaming headphone! It's extremely detailed, has good imaging, good soundstage and is very comfortable. It's also not too bass heavy, which is a plus for specific games. Funny, the imaging and soundstage on the DJ100 and KNS-8400 are about the same. DJ100 just has slightly more forward upper mids.
 
I think tomorrow I'm going to do a comparison between my new favorites..DJ100 and KNS-8400!
 
BTW I can most definitely say the KNS-8400 is WAAAAY less fatiguing after burn-in. None of that piercing treble now, unless the music is normally like that. The bass has definitely improved. I doubt at this point anything else would change, but I'll burn them in for another day or two. The bass at least is now acceptable for me.

Quote:

sophonax said:

Welcome to Head-Fi Byteblock, and as is our customary greeting here, sorry about your wallet! 

Click to expand...

Ha! That brought a smile to my face! Yea, seems every time I let up on it, it is only a matter of months before it starts hurting again.
 
 
Quote:  

sophonax said:

Do you have a Guitar Center nearby?  If so, you can go there and demo some studio-type headphones.  I was able to demo the KNS-8400 and the SRH-840 there.  Being a rather bright headphone, I'd really have to recommend demo'ing the KNS-8400 before buying -- I like these headphones a lot, but the treble could be overwhelming for some.

Click to expand...

Yea, and I used to love shopping there but they've hired some really snooty kids, overly-eager for a commission, and I've been avoiding it lately. I think this is probably a good idea though so I'll put on my white belt and try to blend in.
 
Quote:

tdockweiler said:

I know this isn't everyones thing, but I think the KNS-8400 would make a good gaming headphone! It's extremely detailed, has good imaging, good soundstage and is very comfortable. It's also not too bass heavy, which is a plus for specific games. Funny, the imaging and soundstage on the DJ100 and KNS-8400 are about the same. DJ100 just has slightly more forward upper mids.
 
I think tomorrow I'm going to do a comparison between my new favorites..DJ100 and KNS-8400!
 
BTW I can most definitely say the KNS-8400 is WAAAAY less fatiguing after burn-in. None of that piercing treble now, unless the music is normally like that. The bass has definitely improved. I doubt at this point anything else would change, but I'll burn them in for another day or two. The bass at least is now acceptable for me.

Click to expand...

Your note on gaming is well received here. My DAW rig is also my gaming rig. It is really important to be able to hear people sneaking up behind me in Battlefield!
 
I am also pleased to hear your burn-in is reducing the piercing highs. 

As suggested, I ventured into Guitar Center yesterday. A new and prominent feature since my last sortie was the reek of patchouli oil. This did not mask my own scent from the predatory salesman who I had previously regarded as the most over-eager for a commission. He worked hard for his money yesterday! 
 
I came with an iPod prepared with a dozen songs from varying genres with high dynamic-ranges in Apple Lossless that I had used previously in testing compression formats. I went through about 5-6 headphones. The first pair I tried were the KNS-8400 and they immediately set the bar for comfort. Next, the Shure 840s which blew my mind and crushed my head. I was really impressed with the bass but they were instantly uncomfortable. I don't consider myself a Bass-Head, but the Shure's sound was luscious and inviting. I would have bought them on the spot if not for the intense vice on my dome. 
 
After consulting with 3 of the salesman I tried a series of others looking for a comfortable middle-ground; including a noteworthy Sennheiser pair with silk-lined pads. None of them matched the comfort of the KNS-8400, nor the sound of the Shure 840s. In fact most of them weren't any less skull-crushing than the Shure's. After much unboxing and reboxing I was back where I started between the comfortable KNS-8400 and the bass-laden Shure 840s. 
 
I spent over an hour in there. Constantly switching between the two as the iPod hit on a particularly interesting bit of music that I wanted to hear articulated. Over-all their clarity and reproduction of the sound was pretty much even. I was really leaning towards the Shure 840s though as one of my problems with mixing in headphones is a feeling of mixing deaf without knowing where my bass "really" is. My music is pre-industrial with a lot of influences from aggressive contemporary electronica, and I feel I need to keep track of where my bass energy is at or it gets out of hand in a hurry. 
 
Then suddenly I hit upon an encoding artifact in one of the "lossless" tunes! The vivacious "Vicarious" by Tool, with it's complex arrangement of beauty and distortion had somehow been mangled by Apple's encoder for a fraction of a second. At the time I had the KNS on so I switched over to the Shure to see if I could hear it there. I could, but only barely. These brief few milliseconds of artifact would have gone unnoticed in the storm of bass. Now I was really torn! The Shure sounded great, but were muddying. The KNS-8400 seemed increasingly thin from the back and forth AB testing, but were a source of relief from the Shure's immense pressure.
 
I recalled a quote from Napoleon Bonaparte on the virtues of having a bit of nasal snuff before making any great decision, and did so. Ultimately I decided the KNS-8400 was superior for my needs; most faithfully articulating the sound and comfortable enough to wear for hours. Making my purchase I returned home to quickly do some tests between these and my Mackie HR824 monitors. I was immediately relieved to discover that the KNS-8400 was not lacking any bass at all! I was ABing the cans against the monitors with music as well as my own synthesizer patch-o-doom that explores the bass frequencies with fury and depth. There was nothing missing! I am so pleased! The Shure 840s were not only muddying to the point of not noticing artifacts, they were doing some "bass-boost" nonsense. I expect that sort of thing from entry-level manufacturers, but I am highly disappointed to discover that in Shure. I believe it to be deceitful to market headphones in a Pro Audio context and try to make them sound better than the competition by unfaithfully reproducing the sound.
 
The KNS-8400 on the other hand continues to please. As mentioned here by others the sound stage is impressive. I spent an hour goofing off with a synth patch that routed 2 different LFOs into the oscillator pans, sending the notes as I played in improbable and amusing directions. I am glad to report that the bass is all there through my 5hz-20khz interface, just not as punchy as the boosted Shure.

I got KRK speakers and yes they required a lot of burn-in to get ride of that high harsh sound.

Thanks for the impressions Byteblock, and congrats on the new headphones!
 
Your experience at Guitar Center pretty much mirrors my own.  I listened mainly between the KNS-8400 and SRH-840, and concluded that the KNS-8400 was a little more detailed and resolving.  I was concerned about the comparatively light bass of the KNS-8400, but when I got the KRKs home and listened to them by themselves, they didn't seem nearly as deficient in the bass as they had at first.  Now, I really don't have a problem with the bass quantity of the KNS-8400, unless I listen to it right after listening to other headphones with heavier bass.