After spending some time with the AKG I'm really surprised with it. First I was skeptical of them doing something years after the K701 and it's derivates, even suspected them being a former shadow of themselves (my father had the K340, K501) and just releasing their last best work forever and only being there for the name (for Harman's benefits). I appreciated the K701's technicalities and improvements with later models (I hated the headband) but after I left mid-fi towards HD 800 and Co. they weren't interesting anymore. While I still could hear that they stayed in the game very well it was no comparison. Sometimes I wanted to go back and build myself a sexy headset but the oval soundstage with a weak phantom center always stayed in every model. It just wasn't acceptable anymore to me.
So now after years they bring out their K812 and ******* it's a superb headphone. I kinda understand the exotic "next-gen" remarks that were made in the K812 thread as in easier-to-drive, much more efficient and to be honest I like it that way even when I am curious about the 2014 Violectrics that are coming up. As I mentioned there, future headphones are likely to follow that direction as in getting lighter and easier to drive. It's no threat for the amping world however, they can still continue as before, just with lesser gain in a few cases so all's fine.
It looks like we have the big three again. Before it was K701, DT880 and HD 600/650. Today it's T1, HD 800 and K812 (although 5 years too late) only that the T1 loses out. Totally.
It's a superb headphone and clearly Beyer's best but it got overshadowed by the HD 800 already for good reason, though not in worlds of course. Naturally we'd expect a headphone like the K812 to batter them all after 5 years of work but in reality it doesn't. This can be a dissapointment in an upgradist's eyes but the realist in me says it's all about nuances and incrementals in the high end but still you gotta reach there first (Hey Ultrasone and Grado!). What I can definitely say as a conclusion from my own experience that the K812 deserves to be on the same podest as the HD 800. Yep, that's right. And there it is where they are battling it out to me.
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). 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. Yep. It's pretty exciting for a open headphone with large soundstage to have such a delicate bass reproduction.
Beautiful Midrange: I always loved the HE-500 most for vocals and while the HD 800 did everything right in technical matters (as in authentic reproducing) it didn't have the same emotion but as it's a very neutral headphone it is uncolored and you may not like it. The K812 is similar as in uncolored but I prefered it for voices somehow. Actually I was amazed and expect something going on there. They are authentic as a neutral reference headphone should be (though admittely music is made for speakers) but still a hint more euphonic. Harmonics? I don't know.
High and Details: both are marvellous and at the same time unforgiving, both can annoy you with ancient black metal. Both are not sharp as a T90 or a Pro Ultrasone. 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.
Soundstage: Here is where the Sennheiser still has the biggest size of all while not losing any cohorence. The AKG's is smaller but in no way inferior, in fact it would be the king if the HD 800 just died out. It's that good. No phantom center weakness, superb depth and layering, also having that weird 3d space effect (which I only had with the HD 800) where you turn around and wonder where that came from. AKG practically erased everything that was weak in the soundstage of their old ones.
I could write much more but my English is not good enough. I had some other impressions German forums, I could maybe write up something more fluid sometime.
Anyway, I did some gaming on them and unsurprisingly they were outstanding. Yes, the Fostex rumbles more and hits harder but the K812 is more authentic. I played 20 hours of Borderlands 2 + OpenAL on them and thank god for that game to actually not have bad samples, especially in its music.
Yes, there's the general thought that such headphones are overkill for gaming and I agree on that for sole gaming purposes (that would be crazy) but their technical superiority still can be noticed if the game samples allow.
When the track "ICE" (nice exciting battle music) for instance appeared ingame the MMX 300 couldn't reproduce the bass textures as well, it was more of a 1-2 layer bass getting lost into one note while the K812 showed all layers in perfect balance (naturally the Fostex exaggerated here). Neither could the HE-500 loaner.
All in all I think the ultimate dream of a universal headphone both for music and gaming/movies is the K812 before the HD 800.