What I can say for sure is that using a ten I get the same performance as a 15 or 18, or 21 for that matter, from a considerably smaller box. On the other hand, if the boxes are the same size with equal mouth sizes they have the same radiating area, while the longer horn made possible by the smaller driver and smaller chamber required to house it gives a longer pathway for a lower Fc and a higher acoustical impedance for both lower extension and higher SPL.