Think of the K812 pro as a poor mans alternative HD800. Similar sound and not finicky with amplification and easy to drive with it's low z. The HD800 is another class once you spend the effort to buy a complmenting amp and source. Not cheap. Though the Matrix Mstage is claimed to be really good with the HD800's.
Throwing random tube amplification at the HD800 will also not do any good. Very similar to the Stax O2.
First of all the K812 is the same price as the HD800. Second both can be driven well with an inexpensive amp like the O2 or O2 and ODAC Combo. If you are going to spend $1500 on a headphone go with the well known and well tested HD800.
Perhaps the K712 Pro can be viewed as a poor man's HD800, but they have quite a few key differences.