Reposting this from Head-Fi, in case anyone wants some fairly brief (40 mins listening) impressions from a not-so experienced set of ears 
I was debating whether to post my impressions which I wrote down after the event, as I also got the opportunity to listen at the same time Ra97oR did but with the small Lehmann Studio amp and the Harmon Kardon CDP. Ultimately I was put off because I felt the setup didn't do enough justice to the K812, but since a few people seem eager for extra impressions, I figured why not.
The amp didn't have enough juice to really get a feel for the bass extension, but overall what most stood out was how well balanced the sound was, and how easy to listen to and fatigue-free the signature was. Compared to say, the HD800 driven off a M-Stage, a similar sounding amp, there was notably less sibilance and a smoother treble. The mids do not have the K701's plastic-like quality or shoutiness, have much more natural presence in the lower mid-range, very open sounding and with a great sense of depth and positional accuracy within the soundstage. But like the HD800, they can sound a touch dry, lacking the engaging qualities of the planar magnetic flagships.
From what I could hear of the bass, I found it...interesting. It was almost like a cross between the TH900 bass and the HD800 bass, in that the cup seal design gives it a more focused bass punch than the HD800s, but less than that of the TH900s. HD800 bass sounds very big, all-encompassing, but a little diffuse as a result. Where the bass strikes in the K812s soundstage, it has a more focused, localised punch IMO. It does sound like it has a slight mid-bass hump, and combined with the aforementioned qualities, I feel it may be more enjoyable with pop and modern genres than the HD800. Nonetheless, the bass has a "typical" dynamic-flagship sound, lacking the sense of tactility and solid, weighty fullness of planar bass.
When I first got the K701s the first thing that grabbed my attention was their soundstage, so when I tried the K812, this was the first thing that I paid close attention to. "Well, it sounds like a headphone" was my indifferent reaction to them, as I cycled through the test tracks. Being so used to the HD800's spacial qualities, I find it hard to be impressed by most headphones presentations in this regard. I don't agree with those who say the soundstage is as wide as the HD800s, though its a bit wider than the TH900s, at least on the setup I heard them on. The depth on the other hand is comparable to that of the HD800s, with very nice layering on the z-axis. There is a nice sense of a "filled in" 3D soundstage, without the 3-blob effect or parts of the soundstage missing like on the HD650 and the K701, respectively.
I wasn't familiar with all of the test tracks that were provided (kicking myself that I didn't bring a CD), so I will only comment on those that I am.
1. Eagles Hotel California [Live]-
I skipped through this track initially as I was looking for something special from the K812 to jump out at me. When I went back to it and listened more closely, I couldn't help but smile at how wonderfully this song was portrayed. Granted, its hard to actually make such a great performance sound mediocre or even just good, nonetheless I felt this song flattered the K812 the most out of the test tracks. What stood out was the great imaging in this song (as good as the HD800 IMO) and vocals largely free of sibilance while still being very clear, even when I cranked up the amp past 3 o'clock.
The ambience of the venue was reproduced very well, audience applause was palpable and layered in a very convincing manner. Excellent frontal depth and centre image, you could the tell distance
between the audience and the performers clearly. The drum whacks from the beginning of the song had sufficient body while remaining very tight and clean. Not quite as physical and impactful as the TH900s, which make me nod along with the beat, but the stage here is narrower, less deep as not as open-sounding. TH900 vocals are more sibilant here too, more so than the HD800s (Nonetheless, this song is so well recorded the sibilance isn't bothersome on any of these phones). The only thing to detract from the K812 performance was that the soundstage wasn't as wide as the HD800s. Overall from the headphones I own I'd rank them following on this song (driven from the M-stage) HD800=K812> TH900> HE-500. I feel the HE-500 lack clarity and imaging for this song next to the above cans.
2. Steely Dan-Aja
3. Chuck Mangione- Children of Sanchez
4. Pink Floyd- Time Well, I didn't like the rendition here, though I'd put it more to the mastering of this version of Time. The version I own is the 1973 De-emphasized Black Triangle album, and even then I find the mastering merely acceptable. For Pink Floyd, I far prefer the 1994 remaster of The Wall for headphone listening. The version on the AKG test track has a pretty low dynamic range, I found the ringing bells obnoxious and shrill and placed too close together. Vocals were too loud and forward, probably a result of the compression in this track. I found the treble hard to tolerate, and some sibilance was evident at louder volumes. Its interesting that the K812 won't protect your ears from stridency and mastering flaws, unlike say, the HE-500s which have a way of rounding out the rough edges while retaining the musicality and soul of a performance.
On the Black Triangle version, I enjoy this song on the HD800s more as it spaces out the bells widely enough and wrings out the most dynamics from the track, next to my other phones.
5. Holy Cole- I Can See Clearly.Very good. It was becoming evident to me that the K812 excels with acoustic music and vocals.
6. Manhattan Transfer- Birdland
7. Michael Ruff- Wishing
8. Muse- Madness This is where I thought the amp wasn't giving the subbass enough presence. I don't particularly like Muse or their production quality, but this song was cleanly presented and not irritating. More enjoyable than the HD800s, but not as fun as the TH900s. HE-500s I'm ambivalent about.
9. Kari Bremness- En Elsker I Berlin
10. Concertgebouw Orchestra (Bernard Haitink) Shostakovich Symph No 8 op. 65- Allegro non troppo. Good Timbre and imaging, just maybe not as dynamic and exciting as the HD800s. Again, I wish I had some better recorded & performed tracks that I'm more familiar with such as Claudio Abaddo's Beethoven's 9th Symphony and Guiliano Carmignola's Vivaldi Four Seasons.
The sound is clearly a bit darker than the HD800s for classical, but not HE-500 dark. I'm pretty convinced, even from this brief 40 min listen, that the tonal balance is more neutral than the HD800s. But this does not necessarily translate to a more realistic and impressive rendition of classical music.
11. Berliner Philharmoniker (Saint-Saens Symph Nr 3. Op.78 (Allegro Moderato- Presto - Allegro moderato- Maestoso- Piu allegro- Molto allegro)
12. Berliner Philharmoniker (Saint-Saens Symph Nr 3. Op.78 (Maestoso- Piu allegro- Molto allegro)
