Nice to read more reviews about K812 
Some of your impressions likes mine.
My conclusion  - K812 is a very good headphone, but everybody (before buying) must hear them self and make his own adjudication.

Cucera...Can you list the rest of your equipment?

And what exactly is tizzy?

Tizzy is a word Tyll used for the K812 and it describes quite well the feeling i have with it. But I have to add it is not allways there. It has something to do with the high resolution and slight sibliance.

Definition tizzy: state of nervous excitement or agitation. Like in "he got into a tizzy and was talking absolute tosh"

About my rest gear take a look in my profil it is all listed there.

Wow, great read. I probably will never be able to reach Summit-Fi until at least 10 years from now...it makes me jealous to an extent.

Great review, thanks for posting for us.  I'd love to see a review of the K712 also.

Tizzy actually means a lack of control usually. The K812's IR definitely suggests some nonsense going on with the driver (massive aftershock and noise). Essentially, according to the measurements, the K812 sacrifices diaphragm control for speed and tonality.
 
AKG probably used an extremely thin substrate for the driver, it'd certainly explain the aspects of its sound.

I think it is not so much a lack of control but an extremly powerful magnet (1.5 Tesla) and the fully open built contribute to this impression. But maybe thr higher power of the Bryston BHA1 and Violectric V200 was the reason for the better impression.

...no, tizz is a lack of control, its caused by voice coil wobble and bad modal breakup, and a stupidly thin diaphragm would be susceptible to those things but still explain the stat like detail and air. Magnet strength does absolutely nothing for sound, it only makes the headphone more efficient. 

Oh, and, a current oriented amp (like the BHA-1 an the V200, wouldn't you know it) will improve the control of a headphone, especially a low impedance one like the K812. So yes, that's why they sound better from those amps. However that can only help so much, so don't overstate that too much.

tokato14, where do you got that tech info from?

Various places; a mixture of research, common sense, physics, and experience.

An AES paper would be more convincing, the sound of a headphone is a complex funktion of geometry, diaphragm, magnet...

Um, okay. Think of it this way. A piece of cardstock attached to a tube versus a piece of paper attached to the same tube. Move the tube upwards and downwards and the paper will flex and bend a lot under the weight of the air. The cardstock will move far less, far closer to pistonic motion. The same principle applies to a headphone transducer, the lighter the diaphragm is the more susceptable it is to modal distortion and uncontrolled movement, so the design has to be more capable of compensating for it. The K812 has a small, heavy voice coil and a large, thin diaphragm. That's where the tizz comes from.
 
As for the experience I mentioned, I'll was thinking of two headphones in particular. The KOSS Pro/4AAA has a big dollop of tizz on the treble. The modern production Pro/4AAAT, which has same driver topoplogy and design, has no tizz at all. The only difference is the diaphragm is coated in titanium, which increases rigidity and prevents modal problems.
 
Let me clarify something I mentioned earlier as well: magnet shape does affect acoustic properties and thus the sound. Magnet strength does not.
 
Regarding the amplification tidbit, my friend educated me on that, so I'm not sure where it came from but I know it's trustworthy because he spends hours on researching these things and checking sources to make sure they're legitimate/trustworthy. We're going into the business ourselves later so we're both very well educated on the principles of audio equipment design.

OK I can see where you are going and agree on the membran thing. Good ilustration with your paper.

But regarding the amps I am still quite insecure, because all the amps I used are voltage driven amps and their output impedance is Lower than 2 Ohms and their current abilities should be abundant. Especially the Centrance Hifi M8 and Objectiv 2 should fit the bill. I am an electrical engineer myself (even not dedicated audio developer) and I simply can not explain it.

Low impedance dynamics need a lot of current to keep the diaphragm in check and both the HiFi M8 and O2 are battery powered amps. They simply cannot push that much current, the design prohibits it due to the bottleneck in the power supply. The O2 is a great little amp but it's not designed for high current output, which is why it's not good at powering planars or other low-impedance headphones.
 
The V200 and BHA-1 are both high current designs, which is why they pair so well with the K812. Its also why the K812 gains sibilance when used with low power gear (like the iPhone). 

Great review of the K812 cucera, thanks!  I also have the SR-009, HD800, LCD-3 and TH900.  The K812 is still not in my favorite shops in NYC, so your review will help in my decision to add to my tool box.
For me, the most important feature of an elite headphone is the soundstage.  If it is too congested with all the instruments and voices cramped near the area above my nose and behind my forehead, then I cannot listen an entire Symphony No 9 without feeling claustrophobic.  SR-009 and HD800 place sources closer to each ear and often project them slightly beyond them. Still, without manipulation like using a Realiser or other software, the effect is still in or near the head.  This, including the feeling of the pads on the ears will never mimic the sensation of being in a live performance.
Both TH900 and LCD-3 does something different when driven loudly than SR-009 and HD800 in that at higher levels, the sensation does mimic a live concert.  The colored bass sends shivers and the tactile feelings of room speakers not just on the face but to a real extent, the entire body.  With tube amplifiers, this experience can be repeatable every time.  The problem I have with LCD-3 and to a lesser degree the TH900 is that around the middle of a symphony, they announce their presence as either the pressure (LCD-3) or temperature (TH900) of the pads decide to say hello.
The HD800 gives more space inside the head for each source, but does it at a softer level.  If you crank up the amplifier, the tactile body sensation from the LCD-3 or TH900 just isn't there for the Sennheiser.
Again cucera, thanks for the effort and I look forwards to a listening to a pair of the K812 real soon.

Outstanding work, great review.

Great to read a different take on these. A question: at the end of  Tyll's review he says these are "a marvellously musical headphone for small acoustic ensembles" - that's exactly what I want them for, so I don't think the "tizziness" will be a problem. For that kind of music, especially with female vocalists, I find the HD 800s very detailed but a bit lifeless and the LCD 3s beautifully euphonic but a bit too warm and laid back. I'm deliberately overstating those "defects" because they're both great headphones.  I really like what I think of as the AKG sound:  very dry and airy and with the mids quite forward (from owning the K712s and the K550s). What do think about the K812s  for music featuring small acoustic groups with female vocals - as compared to say the LCD 3s or the HD 800s?

I Would prefer the K812 for small ensemble musik, like Jazz, singer songwriter but for large scale classic musik my choice would be hd800. And the lcd-3 is the best rocker. The only can that does it all for me is my Stax sr-007 mk1 on the kgss.