To keep the cost down I would use $11.25 Goldwood 7 inch poly woofers(fs=35), and use four of them per side in a sealed design, to keep the bass tight. Modeling on WIN ISd, I would need a 1.5 cu ft box for each. To save on weight, I would use 12 inch sonotubes with 3/4 inch MDF end plugs, and lots of fiberglas fill.

Crossover would be at about 200hz. The modeling shows it ruler flat to 40hz, down 4 db at 30hz and down 7 at 25 , which would have to be equalized out(system already has active crossovers and a DOD constant Q equalizer.)

What do you think?