True. RE recommended 2 cubes, Id say 2 to 2.5 though as well. People use to complain XXX's require huge boxes... pretty average sized box requirements imo.
i have run a bunch of XXXs give them the 2.5 cubes and a 32hz tune. 2 cubes is not enough and the sub performs better imo at 32hz at 28hz it loses some midrange impact