Why³. Why did I want two opposed 9.5" drivers not a single equivalent 13.4" woofer? Because two equal but opposing forces cancel. The mechanical forces two identical woofers on the same signal dump into their cabinet null. That makes the enclosure self quieting. Now it needn't adopt heroic construction measures to do a good job at sane volumes.

It's why this layout is called force cancelling or balanced force. Even the boxer engine in my 1200cc B&W Enduro bike during our Santa Rosa/CA days exploited the same concept to reduce its engine vibrations. That worked, demonstrably. By contrast, an earlier Moto Guzzi El Dorado California police bike had shaken my arms like a very mild jack hammer. That's what an asymmetrical single woofer would do to its box.

Once we do extremist subs from 'the big three' speaker firms, we see 158-258kg for Magico's Q Sub in 2×15"/18" flavors; 127.5kg for Wilson's 12" Watchdog which ups to 277.6kg for their 3×12" Subsonic; 111kg-210kg for YG's Invincible in single/dual 21" flavors. Of course those humdingers output far higher SPL without flinching whilst pressurizing even big rooms with true infrasonic energy below 20Hz. Their enclosures counter such ambitious efforts with rude force. That means massive aluminium or composite cabinets. It's when bass gets weaponized. My ambitions are purely civilian not military, then civilized so a good two steps removed.

The low-mass Wilson Benesch 18" Torus walks a different path entirely. In its short carbon-fiber tube, it weighs just 32kg. If my wallet were willing, that's an über sub I'd look into. Back in the real world, today's far more modest 21.5kg Dynaudio built in China seemed quite sufficient for my 2nd-system needs.

Space: the final frontier. There's a fifth and perhaps best reason for wanting a properly integrated 20Hz sub. Anecdotal evidence for this is rock solid, explanation not so much. Owners of subwoofers in 2-channel music systems invariably report the same. Their soundstage enlarges, contrast heightens and perception of audible space—what we might call venue cues—improves. This holds true even for bonafide infrasonic generators in the 15Hz-and-below range playing back a children's choir. You'd swear that the recording contains no true LF information yet even an infra sub still has a demonstrable effect. Nobody who shared such observations could ever truly explain why. Some proposed so-called undertones or subharmonics as an inverted series of the upper harmonics. Now a 440Hz tone would include a 220Hz first subharmonic (½), a 146.6Hz 2nd (1/3rd), a 110Hz 3rd (1/4th) and so forth. To my knowledge, acoustic instruments simply don't generate any tones in a true subharmonic series even though a violin can produce undertones by "bowing with great pressure to create pitches below the lowest open string on the instrument". The tritare guitar apparently can, too. Wikipedia then states that subharmonics can be produced by signal amplification through loudspeakers and that they are "a common effect in digital and analog signal processing like octave-effects processors" and also appear in "subharmonic synthesizers used in audio production and mastering work". Synthesized octave effects I've heard on a track where a clarinet produces a parallel line an octave below what the artist played. It's an obvious special effect well within the scope of a small 2-way speaker, no sub required. Perhaps many recordings simply contain low-frequency very low-amplitude signal of a non-musical nature which adds back venue data when reproduced by a suitably extended very sensitive bass generator?

This ModalAkustik MusikBass sub was set up equidistant with the speakers but started its acoustic 6dB/oct. roll-off at 35Hz. Its timing precision set me on course to duplicate that placement with the 18S.

At the end of the day, I can't shed light on why/how very low 'subliminal' frequencies might embed in recordings which seem to contain no real bass. All I know, as do many others, is that subwoofers increase spatial realism for any music. It's not just about Massive Attack bass orgies. It works for girls 'n' guitars. That's why this benefit is the perhaps best reason to pursue an accurate subwoofer perfectly integrated and set up to time correctly. It adds something more visceral.