How many 6.5in driver equals one 8in driver

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.

I guess it depends on what you mean by “equals.”