The above stated is right. Below 20Hz you won't hear anything because the sound waves are so large. You still need relatively a lot of power to push the subwoofer driver down that deep because it will be moving a good amount of air even if our ears can't hear it.
So I would think that being able to "feel" it under 20Hz at a low enough volume level that you aren't getting a good amount of "above 20Hz" LFE could prove to be a challenge. Also keep in mind that many times the LFE seems louder in a next door room than it does inside the room with the sub. This also happens in car audio where the bass may be moderate inside a car, but outside seem louder. I was told that it was due to how *long* the sound waves are and that they are (in laymen's terms) outside of the car at really low frequescies before being able to be heard.
Not sure if that is exactly true or not, but I can say this; My family will be watching a TV show in our living room where we have a fairly inexpensive Sony HTIB. They won't have it very loud at all, and I get still hear the subwoofer through the wall in the bedroom. The lower frequecies are just harder to trap inside a space. Ever go to a movie theater early enough that the previews haven't even started? Many times you can hear the LFE of the auditorium right next door despite their efforts in acoustical treatments and sound deadening in each theater.
To answer the other questions, use a SPL meter like mentioned above, and if sub-20Hz is wanted, then a large driver/enclosure will get you there easier than a smaller one. I've got a SVS sub that has almost a 4 CuFt ported enclosure with a 12" woofer and it touches 20Hz, but doesn't go below it. Some people have modded their's to do it, but really you would be more successful with a 15" woofer.
So, I guess after all of these ramblings I should ask the question: Why are your trying to get sub-audible LFE at low levels? I mean, that is the "feel it" zone and if you still want to "feel it" then you are going to need to leave the levels turned up a bit.
Or use tactile transducers (bass/butt shakers). You feel it then, even with the audible levels turned down. I had bass shakers in my first theater, and they are back for V4 and everyone loves them. Mine are set to be a little subtle and to augment the subwoofer itself, but they give the "feel" of sub-20Hz even if I turn the speakers and sub down since they are on their own power source and can be turned up/down independantly.
Although, my room is dedicated and fairly isolated - bedroom mentioned earlier where the TV in the room next door is heard even at low levels, and I can barely hear an action movie going in the theater right below - so I don't worry about turning it down much. Keeps the wife happy.
