I have not, admittedly, tried the HD800 yet, but the K812 are the best of the AKG line that I have tried so far [no, I have not heard the K1000, either]. And probably the best "conventional" dynamics I have heard so far.
From my recall of the T1, I think the K812 treble sounds more open. The Beyers had a "hard" or "vanished" character to their treble response that I could not get used to. In fact, from the release of the "Premium" series [DT770, DT800 and DT990] forward, I found the Beyers [a brand I had always revered previously] virtually unlistenable in the high frequencies [with the exception of the DT880/600 ohm]. The last Beyers that I really liked were the DT880|2003 version.
Edited by pataburd - 7/7/14 at 4:45pm