"I'm trying to figure out why everyone thinks they need two 12s or 15s to properly reproduce mid bass in their home?"
among other things, it kind of depends how folks are defining mid bass, how load they wish to play, and the nature of the content being reproduced.
as you know, the spectral content of something like dance music can have 10db+ or more of bass than the spl at higher frequencies.
good, uncompressed, recordings might have another 15-20db of peak-to-average headroom requirements.
we tend to set the volume knob based more on the midrange average spl, so if that is around 90-95db (very loud), a 120db+ system isn't overkill.
as a separate item, there may also be something to the whole notion of sound 'intensity' as well. spl alone may be an incomplete description of the experience.