Well, now I'm puzzled...I thought at least MSI was a safe bet, however they also use that 'OC socket' thing.
I've had my adventures with ASUS's X99 offerings and wouldn't like to gamble anymore on a board. I think pretty much any X99 will max out a 6950X on AIO/ambient if the VRM is kept cool enough? I've played with X99 back when it came out and remember 6950 being a stubborn chip, 4.3-4.4GHz is all they did without having to pump big volts into it.