If you want some amazing midbass, run an array of four Seas Excel 8's on each channel... It'll cost you the same as a single beefy JBL driver, maybe a little more, and will probably sound better. Some of the best midbass/midrange drivers around. Take them down to 60 Hz, couple that with a Brahma 15" in a large, low tuned box... You will have a killer bass/midbass system.

I know my midbass sugestion isn't an 18" woofer, but I just wanted to present some alternative choices to you... You don't need an 18" midbass for a good bass guitar sound. The Quad excels would also give you higher sensitivity or very similar.

Adam