10 cu.ft tuned to 18 hz works well, but as Looney pointed out so does going bigger and tuning lower. Here is 10.cu.ft. tuned to 18 hz compared to 18 cu.ft. tuned to 15 hz, both with 1450 watts input power. The bigger box will give you more low end output for movies in the 14 - 18 hz range. It all depends how much low end output you want. Decide how big of a box you want to go with and we'll figure out the porting.