Gerald,
Where do you find pro cabinets that have a 3dB down spec under 20Hz?? and what signal are you going to send them that's below 20Hz??

Maybe Bag End with the Elf Integrator or a ServoDrive will PLAY below 20Hz audibly, but it more than 3dB below the reference output.

Any club system designer worth his salt would INSIST on a rumble filter or other subsonic circuit to limit signals below 30-40Hz in any system that may ever have turntable(s).
Any record that's even slightly warped will cause extreme low frequency cone flutter at the expense of tons of current that will suck the headroom out of the entire system and cause the drivers and amplifiers to live a stressful and shortened life.

The "boom-shake-a-boom-shake-a" stuff on techno and dance music hardly ever dips below 40Hz anyway, and is primarily 50-60Hz.

A more realistic F3 (or 3dB below reference level) measurement on pro sound bass cabs would usually be in the 35-45Hz range and maybe even higher, but given the prodigious output capoabilities above F3, they will still do fine on the "drops" below 30Hz before the subsonic filter adds to the cutoff.

That EAW dual 18" model linked in a post above looks like nice solution to tuck into a corner for optimum loading.