What I can definitely say as a conclusion from my own experience that the K812 deserves to be on the same podest (at leat) as the HD 800.
Bass: K812 bass is very detailed and doesn't lack the power or "physical presence" like the HD 800's (which is detailed as well but thinner, less exciting as in weaker energy). In no way is the K812 a bassy headphone. In comparison with the Fostex it's pretty clear. No matter how much I love the Fostex though the bass detail really goes to the K812. It's pretty exciting for a open dynamic headphone with large soundstage to have such a delicate bass reproduction. To me it sets a new standard as the dynamic headphone with the best technical bass reproduction.
High and Details: both are marvellous and at the same time unforgiving, both can annoy you with ancient black metal and screeching men with long black hair that want to kill either you, your family or your dog while riding on repulsive beasts from either hell or some northern underworld. Both are not peaky and overstated as a T90 or a Pro Ultrasone is. In no case. That tizzyness is overstated to me (unless you really listen to a lot off mediocre recored stuff, that's where both will dissapoint you) and if you look at the data of my amp you can't have something more neutral than that. And some of you know that I can be sensitive to fatigueing highs as the HE-400, T90, Pro (2)900 and others gave me.
But it's definitely there and sohuld be mentioned. It doesn't have smooth highs like the LCD-3, this should be clear from the beginning. Between the HD 800 and K812 the highs are a toss-up to me as the HD 800 is slightly less peaky to my ears while losing out on detail (slightly) vs. the K812.
Now in a technical sense the HD 800 is still unbeaten in its soundstage. Especially in classical music/orchestra it helps in delivering an impressive huge soundstage that no other headphone can deliver, also not the K812. It's not only huge but also totally cohorent, effortless in its imaging, no matter what you throw at it. Where the K812 is on par and sometimes even better is separation (better), resolution/detail (better to me). Neither does it suffer in cohorence. That's a huge achievement imo, even when it's 5 years later. Who else did it so far? (besides STAX).
Ultimately I favor the K812 vs. the HD 800 due to the bass, and the sharper imaging (midrange is the same to me, though a bit more delicate/authentic, clearer). What I give the HD 800 is the bigger soundstage where it's realistic and a "easier to tame" high level. Though don't think of PRO 900, T90 level here, those are nasty in comparison to the K812.
Build quality wise the K812 is outstanding, something worthy of high end, the material choice and comfort customization is top notch. Neither are there pressure points on the top of your skull (headband is soft and wide) nor pressure/clamp force on your ears. The headphone stand is an Omega in Makassar. It's of very high quality but I don't prefer such a type of stand for ANY headphone. To me the Room's Line is definitely the best as it doesn't touch the earpads of your headphone and let's them dry out. Remember that with every session you wear your headphone you will leave sweat/sebum/oils so letting the pads dry is best and once in a while to wash them carefully. The advantage of good leather pads is a much easier hygiene over time as the leather doesn't soak everything up.
The cable is rather cheap and was probably built for ultra flexibility. Since it's detachable there's a market for custom cables right there.