Fs is not the end all for low end capability, but it is a good indicator for sure. Xmax and woofer size will show you how much air you can move, which will translate to low end capability as well.
FYI There's a few limitations with getting this kind of response
1) physical size - to truely create these frequencies you are going to need a large amount of real estate, most enclosures start at 4-5 CU ft for a 12/15" and 6cu ft+ for 18s. Its going to be a large wood box, and these values are approximates for sealed, ported enclosures will be even bigger. If you have a bigger room you may need 2 subs to get the low end you desire, 4 would be ideal, even I'm not that crazy though.
2) DSP- when modeling for frequenices this low you are going to have issues keeping it linear, DSP is a must. I like REW and miniDSP, I also use a calibrated umik-1
3) power, even with "efficient" subs (90db+) these low freqs pull INSANE current. not to mention a good amount of amps high pass at 20hz!! say goodbye to a lot of information. Even some mfg that claim they can play below 20hz are still attenuated a few db in an effort to make the units more robust. You are going to want to invest into a large pro power amp that likely will have a fan. Many like the crown XLS series or behringer iNUKE, I use 2 QSC mx1500's bridged 1500w @ 4 ohm, 1 per 18"
btw- the iNUKE does have the 20hz HPF, you can disable this with a software mod.
I recommend checking out stereo integrity subwoofers, they all have ultra low FS (<20hz). Their subs are relatively easy to model as well, they respond well to sealed boxes with qtc of .707.
you can grab bassbox pro 6 It's a great modeling program, you can also ask nick at SI his recommendation based on your intent.
http://stereointegrity.com/
I use 2 of the discontinued DS4, its a 18" with 4 ohm DVC 1200wrms rated 4" voice coil. Xmax is 27mm, they are in 6ft cu enclosures with an in room f3 of 17hz. With REW/DSP they are near linear 15-80hz. They produce everything down to 10hz effortlessly, and their implementation overall has contributed more sonically than anything else in my system.
drivers + MDF + DSP will cost a lot.. but the bang per dollar compared to preassembled subwoofers is astounding. I spent near $2500 for everything and I couldn't be happier, I know $2500 thrown at JL or REL will get me nothing comparable