For anyone else that comes across this problem, I just built a new system with the Maximus VIII Gene and got the "EF" error code on boot. I found this thread, and sure enough - the memory was the issue. I hadn't seated the modules properly, removing and re-seating them (being sure to press each end until I heard an audible click) solved the problem.
Thanks to the OP and Sabishii for the solution ^_^