How many 6.5in driver equals one 8in driver

Spandl says: “The trio of 125mm bass drivers give the authority of a single 250mm woofer, but in the same slimline cabinet.”

From Spandl, a designer at Acoustic Energy.

https://www.hifichoice.com/content/acoustic-energy-ae520

+1 @westcoastaudiophile   I have found the quality of the bass from a single driver better than when multiple woofers are used (whether subs or in the same cabinet).  

All of these calculations omit the fact that an 8" driver will typically have more excursion than a 6.5" driver, and it is the volume of air moved, not the cone area, that determines how loud they can play.

If you have multiple 6.5in drivers in series, the cone excursion will be similar to a single larger drivers.

in series drivers will modulate each other due to difference between all drivers involved in Z(f) impedance vs frequency behavior, thus is not common!