I would probably grab those. The fact is 18" woofers are not that cheap or easy to come by. In the music industry they are frequently reconed when they blow, rather than replaced.
You will probably want to build your own cabinets. Probably with one woofer each and you can make a second pair or whatever or flip two of them or at that price keep them as spares in case you blow the pair you use. (ummm, see below)
Some are very efficient. When you increase diameter you increase area as a square of the radius. What that means is that alot less cone excursion is needed for the same SPL. What's more, natural laws of physics say that it will be easier to get really extended low range out of them simply due to the wavelength of a low frequency sound. The diameter becomes less insignificant in compare to the 20 foot long 20 Hz output.
They might not be really high compliance which means you might have to equalize them. however, watt for watt they should put out well over 50 % more SPL for any given cone excursion than a 12". This keeps the voice coil more at the sweet spot in the magnetic field which should result in lower distortion. So do as thou wilt. You can build super small cabinets with a 12 dB crossover and a highly equalized dedicated sub amp and get some damn kickass results.
I would not use those cabinets. Actually you might be able to flip them for maybe twenty bucks at a local music store. Find a bass guitar player, he can get his own woofers. (or sell him two of them with one cabinet ?) Another place might be a bar that hosts bands all the time. In that sort of venue those cabinets will probably sound good, but not so much in your livingroom.
When you get to 18" woofers, cabinets have different needs. What's more is that you do not have access to the TS parameters so those calculators are useless. I would just make a sealed or possibly (small) rear ported design with alot of damping. Might not come out flat but I bet will sound pretty good from the gitgo. Then you mess with EQ and crossovers.
Conceivably, instead of passive crossovers you could use dedicated amps. If you do that, the only advantage to a passive crossover is there will be less distortion when the amps clip. If the amps clip, either the cops are coming or you need more power, because that is going to be a hell of alot of bass. I would opt for the better phase response of an active crossover, but of course an EQ after that.
I don't know if you can get one, but on amps for bass guitars there are EQs tailored for those frequencies. If you can get a pair of them or a stereo version that would come in handy after the crossover. I've not looked in the availability of such a thing but they are probably out there. Also, a 1/3 band EQ would do well enough anyway. They are not cheap but can take care of almost any problems in response.
Somewhere I have an audio test CD with a frequency sweep, if I find it I fully intend to rip it, but then I am sure there is something available on the net. Better yet get a used function generator on "that" site or whatever, they are usually pretty flat at those frequencies. Find your resonances and cut, try to avoid boosting.
Actually, if you have an accurate graphic EQ, once the settings are determined a simple circuit can be designed with fixed EQ.
