You can probably match your tops with a small sub, like a direct radiating LAB12 in a 4ft3 box. It will go pretty deep, the tradeoff is efficiency for size. But if you want deep bass and high efficiency, you have to use a larger bass bin than that.

• Hoffman's Iron Law

Many designers both listen to, and review the CSD(stored energy)plots of bass speakers several octaves above the crossover frequency to see if harmonics, cone break-up, or just distortion will affect the midbass sound. The best designers want all bass "nasties" -60db down, and claim that it is easy to hear even -40db down cone break-up. Very large diameter woofers might require very steep crossover slopes to approach the -60db goal....steeper than LR4 can "ring" and have audible phase shifts.

I have found 15" woofers to have the best balance of bass SPL, bandwidth, and low midbass distortion. The 15" also seems to be the most popular pro-audio size, and hence offers the most options and best engineering.