I'd not noticed this ancient thread before today. Without having ploughed through all the comments, theory and measurement graphs, etc, I'd be interested to go back to the thread title - "S&V Experiment: one 15, two 12, or four 8 inch subs" and ask simply –
If your bass (from main speakers or subs) is to be delivered by one of these driver options, which is likely to be the most satisfactory sound-wise?
My own choice is probably the 2 x 12" drivers. 15" ones tend to be rather "slow" and 8" ones are struggling to reach the lowest frequencies people think they need.
Many (probably most) "full-range" floor standers have moved from large bass drivers to multiple smaller ones over the last 2 or 3 decades, and I suspect this may be why many audiophiles are resorting to subs - a regrettable development in my view. Better to buy speakers with adequate bass up front that don't need subs. Leave subs to the AV crowd!
I've never owned subs, but my own speakers (with the exception of my ATC 50 Actives that I didn't much like) have all had twin 10" or twin 12" drivers per channel, although my very first speakers (home-built Wharfedale Airedales in the 1960s) had single 15" drivers – but these were perhaps their least good feature.
So, what do others favour - single 15", twin 12" (or maybe twin 10”), or 4 x 8"? Just your hunch, no verbose theories please! Peter