To add to what Nick said, I have personally done a 21 in 7.5 cubes tuned to 29 hz, 9 cubes tuned to 32 hz, and 10 cubes tuned to 32 hz. Each time was ran off of one 20.1 amplifier. All enclosures were built with a port area that equaled around 14.5 sq in of port area per cubic foot of enclosure. Out of those enclosures, the 7.5 cube enclosure worked the best. It was probably the loudest of the three, and that 21 liked to play the low notes. This is just my personal experience, and there have been some others I know of who have had great luck using a bigger enclosure, but I dont know their enclosure specs and can only speak about what I have done. Hope that helps.