When using a box with a common chamber you don't need as much air space for each sub. I've found that you're safe with 15-20% less per sub and still able to get the response you're wanting. If you have the space, then by all means go for something like 18ft^3...it'll be more efficient and require less power.Does that spec sheet not say they preform at absolute best in 2.5 cubes each??