Pros: Mids, details, comfort, price, neutrality
Cons: Soft pouch instead of hard case, cable is pretty naff
Comfort and build quality:
The KNS8400 comes with wonderfully soft memory foam pleather ear and headband pads. They can get hot after several hours use, but that's expected for closed pleather. They clamp fairly well, not too tight, not too low, I can have a moderate headbang and these headphones won't budge.
The cable is detachable and about 8ft long(as measured with a 15cm ruler). It's not great since it's very long and prone to tangling itself and trying to coil itself back into whatever way it was last stored. There is an additional 1ft extension cable that includes an inline volume control. The cable attaches the the headphones via a twist and lock 2.5mm jack and terminates in a 3.5mm jack with a screw on 6.3mm adaptor.
The headphones themselves are made of fairly durable industrial plastic with a metal reinforced headband.
I tested this out of my Schiit Asgard amp with a Xonar DX as my source, then out of a Fiio E5 with a Sansa Clip+ and finally out of the Asgard with the Sansa Clip+. All music was in either 320kbs MP3 or FLAC 16/44 with the occasional FLAC 24/96 tune.
One thing to note is that these headphones are resolving enough to tell if the source was the Clip+ or the DX. If your source sucks, these are going to let you know. These have some serious detail, for instance in a FLAC 24/96 recording of Beethovens Poco Adagio, I could clearly hear every sniffle made by a member of the orchestra.
These are kept under control quite impressively and are fairly laid back. This makes it easier to listen to the KNS8400 for extended periods than my Shure SRH840s, which I find quite fatiguing after a couple hours. The laid back sound does mean the highs are less detailed than the Shures. However, the highs are quite sibilant and piercing until the headphones are burned in.
The mids are neutral, bordering on a slight emphasis. There is nothing really wrong about the mids, the KNS8400 does mids very, very well. Vocals sound fantastic and are very clear. Listening to Marina & the Diamonds was a treat on these headphones, although I would say these headphones do male vocals better than female vocals.
The bass is present, but not forward. It exists and turns up when needed, the detail is there but not the impact. It is capable of being punchy, but not of the low subbass rumble or slam you get from speakers or from my Denon D2ks. Good for dance, trance and certain dubstep, but not well suited to techno or jazz. The bass needs to be faster to play well with jazz. In comparison to my SRH840s, they have more punch, but less detail and extension.
It's never going to be amazing with closed headphones and as such the soundstage is fairly average. Classical music in particular sounds slightly congested, but the soundstage is wide enough for anything else. Nothing to complain about, but not great either.
Isolation is pretty average for a closed headphone and the headphones are easy to drive at 36Ω. However they do benefit from some amplification, which helps to bring out the bass and details. Even a lowly E5 does a decent job at amplifying them, but a higher end amp like the Asgard provides better results.
In short these headphones have no real weakness, but at the same time no outstanding strengths other than its detail retrieval and low price.