With a measured 20 Hz Fs/~0.2 Qts, all it lacks is some room/boundary gain and excursion, which is supplied by the extra driver/channel and a couple of corners. Four corner loaded 515Bs do a pretty good job of mimicing a basshorn until you want to start busting drywall seams. Anyway, the system does ~111 dB/16 Hz/200W in-room before thermal power compression sets in, or about what a single driver room sized folded horn will theoretically do at ~2W. My house is a flimsy stick built one with a floating floor though, so they could do at least another 6 dB in a concrete foundation brick n' mortar built one.

WRT its low Qt rising response, this can be either active and/or passively EQ'd. Mine are EQ'd ~flat 20-20 kHz and still close to 100 dB system eff. above ~40 Hz. Not going to compete with the 'big boys', but good enough for my well abused ears.

WRT size, I grew up around huge mono consoles, 7-8 ft^3 end table speakers and 3-5 ft^3 'bookshelf' speakers, so 20 ft^3 seems right reasonable to me for a corner loaded app, especially since they use to be 30 ft^3 before I cut them down. This is the price you pay for having eff. and wide BW.

GM