I got another pair of the 8400's the other day at Guitar Center. I was worried they'd be just like the old pair from late summer of 2010. I risked it since they don't accept returns. Katun and some of his comments made me want to give them another chance. They were always good, but the 6400 impressed me more out of the box than the first 8400. I actually planned on getting another 8400 anyway since I always liked it.
Please take all my comments with a grain of salt for now because this is based on under 3 hours of listening so far. I haven't done any side by side comparisons yet.
The new 8400 I got in seemed to have a lot better bass. This was immediately noticeable. I don't know yet how much it compares to the 6400 in quantity YET. My first 8400 was nearly bass light, but not this pair at all.
To my ears the KRK KNS-8400 seems to have a LOT more treble than the 6400. I believe Katun felt the 6400 did, so it's best to hear for yourself. It could be a combination of my cables, sources and Headroom Micro Amp.
Very brightly recorded and harsh music is still bothersome, but it's not the headphones fault I think. I will burn the 8400 in for a few days. I described my first pairs treble as like icicles stabbing me in my ears. This pair is NOT like that even remotely.
I imagine after burn-in the treble will smooth out a little.
On my old 8400 the soundstage was not that good at all. This pair was a noticeable improvement over the first pair. Night and day difference. The 6400's soundstage was WAY better than that of my first 8400 easily.
The 8400 seems to be noticeably more clear than the 6400. I still think the 6400 is not that far behind in this area AT ALL.
What's super surprising about this 8400 is that it has TONS of detail. Possibly the most detailed headphone I've ever heard. It might even make the DT-880 look bad in comparison when it comes to detail. I'm going to compare the 6400 vs 8400 for a few minutes and see if I can hear some detail that's not heard in the 6400. They'd have to be very very tiny details that would not easily be heard. I know for sure the 8400 can pick up recording hiss and background detail a little bit better.
One thing I need to compare is how good and accurate the soundstages are. Not when it comes to size, but just how it presents it. So far I'm pretty impressed with it's soundstage and imaging. It's up there with the K601 when it comes to being able to pick out where all the instruments are coming from. I'm pretty positive it has the HD-598 beat in this area.
The differences between my two pairs is not HUGE, but enough to make me like them a lot more. The treble is still a bit bothersome right now, but only fatiguing with treble heavy music. I'm sure it will smooth out. I need to try various amps and see the results. If you love a lot of treble, the 8400 won't disappoint you!
The 6400 to me with my amp seems more natural and balanced. I figure some won't agree with me at all. I think Katun said he felt the 8400 was more natural sounding. I'll see if my opinion changes when I do some comparisons. The mids definitely seem more forward on the 6400, which is what I like.
Since I like my headphones to be great for female vocals I will compare them both for this and see which one I prefer. Maybe it will still be the 6400. The 6400 is just so much fun to listen to and I love it's mids and the treble doesn't even remotely bother me, but it does a tad on the 8400 (so far, no burn in yet). Again, I'll check my sources, amps and cables and see if the treble is being made worse somehow with them. Maybe I'm just super sensitive to treble, but I don't think so. It's not that bad really at all. The treble is barely an issue.
Going to do some more comparisons with my DJ100 now that it seems "fixed" with burn-in. Maybe I'll finally have a headphone that tops the DJ100 as a favorite.
BTW the amount of treble reminds me of the K702. Both were capable of bothering me a bit. If you're tried the K702 and have zero problems with the treble, then the 8400 will be just fine. I think though that the 8400's treble is much easier on the ears compared to the K702.
On some songs I think the recording is just garbage and made brighter than it should be at the studio. Sometimes it's hard to know for sure if it's the recording or the headphones fault!
EDIT: I'll add a big note to the top of my old review of the 8400. The differences are not massive, but enough for me to be even more impressed. I definitely like the new 8400 more than my first..no doubt! I don't know for sure if my old pair was a lemon. It could be just an early pair that didn't sound at it's best. Sometimes different pairs of the same headphone can vary. I always have this happen. Maybe KRK made some changes to the headphone after the first few months of release. I doubt it. Sometimes amp synergy can have a lot to do with how a headphone sounds. I remember trying the first 8400 on different amps to see if it's sound changed much. I used an Asgard for my first 8400 I believe. I no longer have that amp to try out.
I might post some more 6400 vs 8400 impressions when I compare them, but it'll just be a few short notes and not a review. I'll do it after some more burn-in of course. I really want to compare them to my DJ100 and hope the KRKs lose badly when it comes to sound..probably not likely
EDIT2:
I'm comparing the DJ100, 6400 and 8400 now. I "MIGHT" be totally wrong on the 8400 being more detailed. Maybe with my excitement I overdid it. We'll see soon. Clarity on the 8400 seems maybe a tad clear. We're talking like maybe 2-3% more clear. I'm absolutely 100% sure the mids are more forward on the 6400 and it's so nice for most music, especially pop and female vocals. For just plain listening to music and not for sure as a studio monitor, the 6400 might be more fun to listen to. I'll know soon. The shocker is that there is detail I can here on the 6400 but not the 8400 until I boost it's mids by 6db!! Want to know what it is? Rustling wind in the background that's ultra subtle. Yes, I'm a nerd I guess.
To me, the DJ100 is holding up very, very well. I haven't found a single thing on the 8400 that my DJ100 can't pick up. NOTHING! DJ100 has more mid-bass too. I still get the impression of more bass impact on the 6400 compared to the 8400. I'll be testing them for quite awhile. Seems the 8400 feels more like a studio monitor and the 6400 more like one that's a bit more fun to listen to perhaps.