Here's the problem today:
Most people don't want to overwhelm their listening area with speakers. So they buy small satellites and put the burden of the critical mid and low bass to the so called "subwoofer". I blame this on Dr. Amar G. Bose completely for in the 1980's of coming with this silly acoustimess system using bandpass boxies.
Now if you have DECENT mains that can produce really good tight snappy bass to at least 50-60 Hz then a subwoofer can do some good. Really two or more are nice (unless you're in a 10x10 room hehe) as the more you have the better. If the driver(s) are moving 30% of xmax at the highest "sane" position you should stay out of the danger zone (distortion wise) when the insane stuff is called up.
Really expensive mains can produce an honest 20Hz but these are costly AND require an equally costly amp and circuit to drive. But it's still possible to produce 120 dB SPL across the normal 20-20k range without breaking the bank. But it may look ugly.
And for those saying 120 dB SPL is overkill or not needed I suggest you head to a drum room and listen to the snapping of a snare. The key here is dynamic range. Of course if all you listen to is pop it really doesn't matter now does it?
Of course I'm biased.
