Speaker cabinets for bass guitar are (or should be) full range speakers. They are generally tuned to drop off after 31 hz, the lowest note on the 5-String bass, but, depending on the configuration, they may drop off as high as 44 hz. They are full range speakers because of the wide frequency range produced by the instrument’s voicing and timbre. They also are not designed to rely on room coupling.

Getting an 18-inch driver doesn’t mean you’ll get better “low note” performance than with a smaller driver, say your 15s. While it has the surface area to reach lower, it’s loose and lazy and can’t provide a lot of definition on its own. The 18s are rare for bass guitar speakers because they are too slow. Since they are usually direct, they are more useful for larger rooms where you get less room coupling and need a larger “throw”. I think cabinet design and tuning is much more important than driver size anyway.

If you’re interested in the subject of bass guitar vs. subwoofer speaker design, browse through www.acmebass.com. The 2x10 and 4x10 can be bought as subwoofers for PA applications.

I'll try running one of my 210s instead of my M&K VX-100 (when the wife and kids aren't home) and report back here. I've always wanted to try it but I never got around to it. Now I have an excuse.