In my perfect world, every Speaker would be built on a down-firing 18" Subwoofer!
To be fair, I should specify that the difference between a 1-, 2- or 3- matching woofer design is as I described. This different than a 2-way or 3-way Speaker.
You can put 4 ea 4-Ohm 6" drivers in series parallel and maintain a 4-ohm load, you will gain SPL, but you will not increase extension.
Taking any speaker design you want to come up with, say a 3-way with a tweeter, mid, and 6" woofer... the only way to increase the extension would be adding a larger driver with the appropriate cabinet volume (and port tuning if vented) necessary to achieve the potential of said driver.