I have really been enjoying the KRK KNS 8400. Like another user said on this forum; I do find that you have to fidget a bit in order to get the cups at the right position. The soundstage is affected and so is the sound, being a bit forward. However! This is not at all a big deal as you can find the comfortable position in reasonable time. It is FAR from being a deal breaker.
I enjoy how clean these sound. I am using a Clip+ (Rockboxed) so I understand that the potential of these cans is not being reached. Regardless, they sound great. I much prefer them to my far-more-expensive Pro 900. The very neutral sound is presented well and I also find the sound stage, while being a little less expansive in width, depth, etc, is a lot more realistic not only in it's instrument placement but the general sound scape created. I felt like the Pro 900 was putting on a bit of a show with me. Not just with the absurd frequency response. The sound stage was often expansive but very unrealistic sounding.
I find the 8400 to have a very clean sound. The bass is lean and definitive. Perhaps a little light? Maybe, but the definition and lean nature of the bass overshadows the possible lack of it. I can hear all kinds of detail and texture in these cans. Again, maybe a little less than my Pro 900. But I am not getting nasty sibilance and the higher micro-detail retrieval of the Pro 900 was often overshadowed by a really boomy sub-bass. It obscured stuff to a point that I had to utilize a parametric equalizer to great degrees in order to hear detail. Consequently, the sound signature was just ugly and jagged - but clear.
I notice the build quality of the 8400 isn't exactly fantastic. However, the memory foam is a really, really, really nice touch and the 8400 is as light as ever. They are very comfortable. I sometimes forget they are on my head.
More about the sound... They are tight. Again, maybe not as tight as my Pro 900 but they are still very tight. I can hear lots of details and they are presented in a somewhat laid back, not in your face manner. But this isn't to say that they are unrealistically laid back. Often I think they sound a lot more realistic. They aren't trying to wow you. They just want you to hear the recording. I often hear the flaws in metal recordings. By the same token, I hear the subtle details in well recorded music.
Are there better headphones of this variety out there? I don't actually know. I have never had any first hand experience with other cans. However, I don't doubt there are better cans out there of this type. Regardless, for the price these headphones are awesome. Actually, I am really enthused about them. I love them. They do sound really great and they are giving me what I want - neutrality and ultimately fidelity.
I am sure they'd sound better with a cleaner amp and source. I just don't have the finances right now.
Would I recommend these to others? Absolutely. Albeit I have limited experience I am a musician and do have some idea of what a proper sound is. I think these cans are excellent. Are they perfect? No. But they are pretty damn good.
Oh! One more thing. They lack sub-bass. But I am of the opinion that they are more about detail and accuracy and leanness, not sub-bass; a quality truly native to headphones. They don't seem to be trying to be something more than they are... I theorize that KRK decided that they would work within their limitations (that is; producing headphones) and not try to be something they are not - like loudspeakers and a sub-woofer. This doesn't mean the bass loses all intensity. Actually, I'd say that when headphones have sub-bass details are obscured. I'd rather sacrifice sub-bass (an unrealistic headphone trait) and have quality.
My Q701's signature seems a tiny bit smoother, but very similar. I do think the 8400 has more treble.
