HI Jim,
A horn simply makes bass differently than a bass reflex. If properly designed, I seriously don;t think ther eis much difference.
If you want to extend the bass of the driver (like in lowther) the horn may be the way to go. You will get some coloration with a horn.
With a bass reflex, if poorly designed, you will either get no bass, or a dip in upper bass. If well designed, you will get adequte bass, but not as much as a horn. However,, you wil lalso get better coherence ( i think tighter group delay) than a horn, since the sound is not travelling thrugh a tube. IMHO, a lot depends on the driver you select. I'd rather select a good driver which will give decent bass in a bass reflex, and do a well designed bass reflex box; than select a driver doped with magic dust and then flog the bejesus out of it by building a humongous horn.
Just my 2 cents.
thanx
-akhilesh