i got my fu-18 in a 7.5 cube box tuned to 45 hz, 150 sq inch of port.... it actually sounds nice but there are some notes where it makes the sub sound sloppy, like really low notes, rougly around 15-25hz it is just slopp... but then everything else and up sounds preaty good.. all i gotta say is that this sub can get **** loud... but i think if i waxs to build another box i would make it either 6.5-7 cubes still tuned to 45 hz, but with only like 100-120 sq in of port.... i think that i would prob get more output out of that....