awhile back my buddy talked me out of a set of a-5's for his sound company here in western montana.b he'd never heard anything so clean and loud ever. the problem was that his current subs were rendered ineffectual. couldnt hear them at all with the altecs running. hehehe. so after selling him on the concept of high eff. horn loaded speakers, the next logical step was good horn loaded subs. after old guy recommended the tuba for the 515g, we settled on the titan for volume..spl. i wrote to bill fitzmaurice, with no reply, about wich altec driver would best be suited for this cab. all it says in the plans is..that any driver with an fs. between 35-50 hz and qts between .3 and .4. fairly vague... the throat on the baffle is 50 sq in, horn length is about 11 ft , horn mouth is ~ 6 sq ft. im going to try a 421-8h in it, altec used it in the 1215. also the 515-8g, 418-8, and a 3156. my buddy used emminence definimax 15's in his 4, still got 4 more here in the garage, ive been sanding and painting on just one of them, piano black, to test in my system, in comparison with my 6th order banpass. still looking for the best altec sub 16- 80hz