Agree with @douglas_schroeder that listening to a system with strong output down through 15 Hz is a completely different listening animal than most people would suppose. Whoever said that there's just not that much musical info below 40 Hz was absolutely wack.
Also recall that "20 Hz" does Not represent the "lowest sound we can hear"...it simply is the lowest sound we can hear that most people can Identify as a **musical pitch** or **tone**. But, there is all kinds of musical information - having to do with tone, dynamics And spatial relationships - that can easily be felt as well as heard. Not to mention the idea of properly pressurizing a room of a given volume vs amp power vs typical listening levels.
But I certainly agree with others that, more than anything else, it really doesn't just take a custom system (which indeed it does) but it takes a truly custom room. A nice, reasonable, typically good set of dimensions might be on the order of 25 or 30 ft long, by say 20 ft wide and at least 14 or 15 ft tall(!) And kiss goodbye any thoughts of 2x4 construction - you'll need more like 2x12's, instead - floor, walls and ceiling alike. And you will still need room treatments, maybe not as much as in a 'bad' room, but you'll still need them...and EQ.
People do engage in these sorts of projects from time to time, but all that is a major barrier that most people aren't quite willing to try to scale for obvious reasons. The rewards are there, particularly if you're looking to build your next home, or modify you're existing house (if you've accumulated home equity over the years, that can come in handy). True, full-range sound can be had, but, done right, it will come as a nasty sticker shock. But impractical problems sometimes require just plain impractical solutions.
