I've been wondering why drivers don't come any bigger than 18inch (45cm). Is it because the cone tends to break up? The magnets get too heavy? Too hard to fit in a box for shipping?
The Dayton DCS450-4 is one of the biggest drivers available on the market, and has a 1W efficiency at 20Hz of about 95dB (supposedly).
My plan is to drive it in a small box (small in terms of the resonant frequency) so that it operates entirely below its resonant frequency. This makes the curve smoother and the resulting 6dB/octave efficiency tradeoff can be corrected electronically (at a cost of high power for very low frequencies). Useful output at 4Hz is practical with this approach, without requiring an enclosure the size of the loungeroom.
However, I'm going to use sonotube for the enclosure, so I'm tempted to buy a few and make a transmission line design for shits-n-giggles. I'm going to need perhaps 4m of tube - luckily home depot loans large Utes for $20/75mins.
Rob Elliott says that the test of such a driver is whether you can put a few hundred watts of pure 20Hz through it without hearing harmonics. My amps (chipamp.com) can only deliver 100W (running in bridged mode), but in this test, despite the cone leaping back and forth no sound was noticed. Yay!