size of the driver

Drew, I agree more than disagree. Dispersion can definitely be modified with waveguides and phase plugs, and beaming can be mitigated with 1st order crossover slopes. Large diameter woofers can be just as fast as small ones if they have the magnet strength and cone stiffness.

Still, if the issue is why 5.25" and 6.5" woofers became the de facto standard over the 12" woofers of the '70s, it's probably a combination of manufacturing costs vs. performance and WAF. The old oversized bookshelf speakers of the '70s had no WAF whatsoever and the bass extension (especially of the JBL L100) wasn't so great either. The PSB Synchrony 2 is sexy with a narrow footprint and has flat response down to around 36 Hz.. 12" woofers can kick ass, but not in a 2.4 cu. ft. enclosure; a pair of 6.5" (a la ADS 810) do better in that same oversize bookshelf enclosure; many 12-inch woofers need about 4.7 cu. ft,, at which point you'll hear some *serious* bass.

If 12" drivers were as fast and accurate as 4" drivers we wouldn't need any midrange drivers.

You can add 6dB for a floor mounted woofer (as in many 3-ways), 6dB if there are a pair of bass drivers, and 6dB at the cross-over point to a sub-woofer....

Drew, why are you adding 6dB? It's power output, you should be adding 3dB in each case.

Back in the late 70s when i sold gear at tech hifi, bass drivers in most
speakers back then tended to range from 8 to 12 inches mostly with the
occasional 18. I levitated towards smaller drivers mostly for better sound
for most with their amps in their applications. Larger drivers were largely
inadequately built and under damped in many cases i heard resulting in a
muddier sound although typically with more lifelike impact. .

I think both amp and speaker technology has come a long way since and
that those issues can be adequately addressed these days in a variety of
ways, with both larger or smaller drivers, if the overall design is good.
Often though size, large or small, still matters though to many sound
quality being just one of the important considerations.

real bass comes from moving a large surface area gently -not a small surface violently . 6.5" is not a woofer size IMO