How many 6.5in driver equals one 8in driver

Just because a smallish woofer can theoretically put out a 30 hz note does not necessarily mean that it actually sound big & full in the bass in a speaker like a 12” or 15” driver in a bigger ported speaker can that falls off quickly after 35 hz . The smaller driver has to move faster & further than the big driver to produce low notes at higher volumes which can & does introduce distortion of various kinds as the voice coil heats up & changes impedance. 

You also have to account for xmax, enclosure design/ size, efficiency, etc. if you're looking for comparable performance between speakers.

I have a set of Snell Type B Phantoms that have dual 8 inch woofers. They provide very decent bass. However........I previously owned Snell Type A V5's. They had two cabinets with 18 inch woofers. They could shake the house if called upon. I will never be satisfied with anything less! The Type A V5's were Stereophile Class A rated. It would cost well over $60,000.00 in todays money to replicate them. I sold them for a paltry $2200.00!!!! No takers otherwise. It is a very strange world. Very strange.

there’s no replacement for displacement 

Probably an easy question but the answers are even more complicated.

Larger bass drivers are always better but the audio manufacturers went to smaller sizes about a dozen years ago because people wanted to fit their speakers into their rooms and because it was an opportunity for manufacturers to begin selling subwoofers. Subwoofers after all are meant to deal with inadequate main speakers it's that simple.