I came from the AKG K712 and since they already opened up a whole new world of sound for me, I did not expect that much of a surprise. Sidenote: I am using a Steinberg UR44 with the headphones plugged in there, directly. I have not tried a separate amplifier for headphones yet. It is the first high end headphone I tried so far. I am mainly working with EDM music and sometimes listening to classical tracks for which the K812 seems to be designed.
The K812 exceeded my expectations. They did it so outstandingly, I could not believe my ears. Out of a sudden, I was able to hear everything in a mix so crystal clear, as if I could just grab the sounds one by one. The high frequencies are very clean and clear as well. For the first time, I was able to clearly tell if a mix was ruined by the high freqs or not. Though, there is a slight elevation of different freqs at around 2khz, 6khz and between 8-12khz which may be fatiguing over time. Because of that I am not totally able to mix only by the headphones which is a bit of a let-down as they are performing so well.
Therefore, I would recommend to use the headphones for just listening to music or as reference headphones with focus on the mid and lower frequencies.
-Huge and clean Soundstage
-clean (sub)bass area
-very comfortable for endless sessions
-overall superior sound quality
-elevated high frequencies therefore sometimes fatiguing