I'm hiving this off from another thread, as I think it's a great topic itself.
Originally Posted by: jgauker 
Our bass player uses a Low Down 400 head with an old cabinet with a generic 15" speaker and it sounds pretty good to me. ... But I still think he could do better with a different cabinet. But I've always been one to like the tighter low end that 10" speakers give the bass. 15 and 18 inch speakers seem to add a little mud when used with bass amps. I honestly think the are better suited as PA speakers.
At a repair shop (south Snelling, St. Paul MN), I once had an opportunity to buy a 30" speaker.
Okay, it woulda LOOKED cool as all heck, maybe hang it behind someone's service counter as a display piece... but c'mon.
(I've since heard that the huge one-piece paper cone was prone to flutter at certain sustained frequencies, which would suddenly shred. Hmm: maybe metal cones...?
)
There are many 15"-18" speakers that are billed as "PA/bass guitar" by the manufacturer. That seems to be a holdover from the days before everyone & his dog had a purposed subwoofer, something specifically intended to reach those tones at & below the bottom of actual hearing.
If I was to mess around with a 15, I'd set it up as a biamp or even triamp, maybe cut at 500 Hz, so that it could stick with the super-low tones & not have to struggle with mids & highs, which would likely clean it up... but that's really below anything musical & well into the visceral -- that is, you
feel it in your feet & guts, down with a padded kick drum.
For bass guitar, to get a good thump without so much vague thud, a 2-12 or 4-10 seems to be plenty sufficient. For large venues, just double the cone count. Even then, many players will benefit with a 2- or even 3-way cab -- a horn for rich mids & a whizzer to access the snap.
Putting a cab on a guitar is generally MUCH easier than for bass. A bassist needs to dig down toward subsonics, yet (say for jazz & such) wants to both access the middle of the sonic wall to have some small presence with the other instruments, & to get some shimmer (particularly for slap/snap playing). Speaking as a guitarist, it's cool to be able to shake the floor, but that really is the bassist's job, especially in recording.
Cone size aside, my experience is that a proper bass-guitar speaker should have at least a 2" coil, & very long throw. (Don't quote me, but I'm pretty sure the 30" had like a 1.5" coil, which just screams "PA speaker.")
One reason I'm drawn to the Avatar loaded cabs is that they custom-ordered Eminence Delta LF12 speakers with bigger magnets & longer throw. And their 15" cab is a three-way!! A 6" paper-cone mid, & the big cone gets everything below 800 Hz, & the box is specifically tuned for those low-lows.