They use a PA
David Botrill, who engineered and mixed and produced "Aenima" and "Lateralus" gave his secret in an interview in "Tape Op. He gets the massive Danny Carey drum sound by spiltting the tom, snare, and bass drum mic signals, and running them through a very well EQed PA system set up behind the kit for the plethora of room mics to pick up. Power to spare, esspecially with Mr. Carey playing!