AGoon -
Depends on the Fraction. ;-)
For me, the path has led to parts from Bruce Edgar, vintage drivers (RCA, JBL) and modern ones (TAD, fostex tweeters), along with vintage woofers (Altec, EVM) though modern TAD's may work there too (1601C I think...?). I suggest active Xover and multi-amping as the way to go though many also like passive Xover and that is fine too.
The next step up is horns from Sierra-Brooks and maybe more exotic drivers, Field Coils (speakers and drivers), though I have not gotten that far yet, budget or time wise (to find the parts). Many have, however.
Beyond that there is no limit. ALE and Goto spring to mind as the ultimate driver, though you can pay up to $40-60k for one of their bass drivers w/o horn, so that eclipses the AG stuff by a lot, as far as price. but, everything above other than that level (ALE, etc.) is less than the AG's and better sounding to me (and many serious horn afficionados.) Check out some of the wild stuff the Japanese have done in this regard (for many years)... DIY horn systems with Basshorns built into the house, with whole walls of a room being the mouth of the horn...
-Ed
