Also in my experience long listening sessions with T1, T90 (extrem sharp highs), DT 880 or HD-800 makes my ears hurt, but for the first 1/2 hour they are ok. With K812 Pro and Stax isn´t this a problem i can hear both over many hours without any problem.
I found the T1 and many Beyers to have a big spike in the upper treble, to me this comes off as brightness and sharpness, other people don't even seem to notice this and say the headphones are on the dark side. The HD 800 was just plain bright to me with an emphasis in a larger region of the treble than that of the T1 so it fatigued me really quickly in only like 10-15 minutes, some systems I could listen to the HD 800 longer but never much longer than 30 minutes. I'm still waiting to hear the K812, I have heard the bass heavy K1000 and was thoroughly impressed, I hope the K812 impresses me as well. It seems the K812 is picky with systems like basically every other AKGs.