nopiano said:
I suppose this is one way to learn that efficiency and bass extension are trade offs, one with the other! Now you’ll be able to use a 3watt tube amp!
And of course, enclosure volume.
Consider a horn loaded 15 inch driver, for a cut off frequency in the range 50-60hz, the horn needs to be about 12-15 ft long with a diameter around 12 ft. Picture this, it is actually quite large.
For practical applications, the horn is usually folded and designed to make use of room boundaries, for real world applications look at the Tannoy GRF enclosures or the Klipschhorn, both designed for corner mounting. When mounted away from from a room corner we end up with the classic JBL 4320 or Vitavox Thunderbolt enclosures which are actually useable, check out the big Living Voice systems as examples of Thunderbolt systems in use.
These are still pretty big enclosures, typically 16-20 cubic ft, and remember, this is the bass enclosure only! Back in the '60s and '70s, JBL optimised its best 15 inch drivers for reflex loading, into 9 cubic foot enclosures, typically 3 ft tall, 2 ft wide and 1.5 ft deep, the basis for the 4320 and 4330 series studio monitors and the hi-fi equivilents, the various L200 and L300 models.
As far as I know, the L200/300 models were, until quite recently, one of the last 'bin and horn' models build for the hi-fi market by a major manufacturer, the revival of the Klipsch Heresey/Cornwall models in recent times being one of the few exceptions I can think of.
As far as the Yamaha system is concerned, it would benefit greatly from a larger bass enclosure but that will involve a lot of work, interesting project though.