Low frequency response needs size. If you're willing to cutoff at 200 Hz then little stuff will work. In my high school years I built a stereotypical mono subwoofer of the 2x4 sheet of plywood for front and back variety. Sounded pretty good, but could have been better.

A mantra thats repeated continuously, mostly by people selling mono subwoofers or uninterested in audio reproduction, is low freqs are non-directional.

Yet I go outside and listen to thunderstorms and passing unmuffled motorcycles and firearms and much bigger .mil weaponry and it sounds highly directional. I can point at the rumble of distant artillery as well as any other sound.

Surely if you build a directional subwoofer system and can only perceive a field, that'll work OK even if its an economic financial fail. But the other way around is an epic fail. So save money at the risk of sounding bad. Well, if the point is spending money to sound good, having an extra 3 dB of signal is never a loss...

Another puzzle is I know from experience that L-R and L+R sound different from FM subcarrier detection. Supposedly mono subwoofer people claim that magically only applies to higher freqs. I would think a single mono channel subwoofer would sound "better" fed off only L or R channel than some peculiar mix of unknown phase. The problem is which channel to connect to?

Phase relationships are a pretty big puzzle in audio once you get past simple response problems and simple distortion problems.