Unfortunately I haven't heard either the HD800 or any of the other x70x AKG line yet. I hope to hear the HD800 soon and I'll most likely purchase it at some point down the road. I'm thinking about demo'ing them along with the T1 and possibly some others, through the Cable Company.
On the K712's, I would point out that I demo'ed the TH-600 a couple weeks ago for a few days and got to know them pretty well. While the K712's soundstage is definitely impressive and a breath of fresh air coming from the slightly narrow-width stages of the HD600 & DT880, the depth of the K712's soundstage still isn't up to snuff compared the TH-600. Man, I really loved how those headphones stretched out the center image in front of you in a very natural way. I suspect angling the drivers of the K712 might give you that and I suspected that would be one of the key points of the K812 but from initial photos of it the drivers do not look angled which is a bit of a disappointment but everything about that headphone is still up in the air so who knows.
I do love the K712s! A lot. Balanced sound/neutraility, comfort and breathability, transparency, and detail are always my main concerns and the K712 fits the bill really quite well. I don't regret this purchase at all. I'm looking forward to giving more impressions once I stop enjoying them so much and start thinking more objectively about them haha.
Edited by devhen - 10/4/13 at 12:34pm