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.
You missed out what I was saying. Yes they are the same price but if you're planning to buy and subsequently own the HD800's then prepare to spend big $ (also subjective amongst users) to get them to sound its best. It is one of the few headphones in the market that one needs to pay attention to with amplification.
Your second statment is a bit silly, even a Cmoy can drive the HD800's. Not properly and not to its potential, far cry from it, same goes to the O2 which sounds like absolute crap with the HD800's. Been there done that with the HD800's 3 times, low and high end gear.