You can also use EQ.
But build a spacer to move the woof back from the cab. Otherwise the cone will hit. Ask me how I know.
Simple with 1/4" ply and a sabre saw.
A7's will go low with EQ.
Otherwise add subs.

Here is a great one!

http://www.altecpro.com/pdfs/vintage...%20Speaker.pdf

I can't find the PDF for the system.

Ron