Learned a long time ago that at 100 db in an organ chamber I still could not hear around 20 HZ. Oh I could feel something but I couldn't hear it. Some where around 24 hz which is close to the note G. A couple of notes below the reach of Piano I could definitely perceive sound level.
So on my HT processor I cut off everything below 25 hz, which happened to be the limit of my front speakers in my room anyway. No use damaging speakers or waisting power on some thing I can't hear or can only slightly feel. From many years of installing discos I learned the 1/3 octave below 40 Hz was all you needed to excite dancers legs on the dance floor. Get that feel in the gut and chest takes a mighty big strong system. What you need to do is add up the total area of your woofers and estimate their displacement at 80 hz with the power you have. Then figurer you need 4 to 8 times that displacement to produce 20 HZ effectively.
Some folks like to raise the band below 70 HZ or so 6 db to get that extra kick. lets say you have 3 12's and 2 8 inch woofers that can move about 1/4 of an inch at 80 hz, that would be 108 pi+ 32 pi X.25= 35 pi cu inches. If you were conservative that would mean about 140 pi cu inches volume of displacment for the subs. If the sub could move 1 inch, then 2 15" sub woofers would give 100 pi cu inches. Two 18 inch subwoofers together would give you 160 pi. So are you ready to go buy a professional JBL sub system with two 18 " subs and a crown power amp with about 1000 watts? I don't mean JBL PA 18 inch type woofers. I mean their special long travel voice coil sub woofers that will reach down to 16 hZ or so. The amp you can find cheap, not the woofers. They make a dual 15 that does really well. If you can afford a Maserati, I guess you could choose Magicos dual 15's with built in 2000 watt amp. If you have a corner available you could pick up some significant level to reduce the power requirements and possibly change from two 18's down to one, or possibly one long thrown 15, but I would not count on it.
There are very few sources that reproduce below 16 hz, except maybe a turntable with a lot of rumble. I was entertaining back in in the 60's & 70's watching Mac ML-4 woofers and Concert Grand Grill cloth flap around caused by noisy Reko-kut, Thorens, Dual and other puck driven turntables. No wonder Empire belt driven tables and the mighty little AR Turntable became so popular with very low rumble. AR 3 owners would be in all sorts of distress when the woofer cones struck the grill cloth.