A 4 string bass guitar's low E string is 41hz, and a 5 string bass low B is about 31hz, and a lot of ambient, drum tone low frequency and other normal stuff exists below 40hz in music. A good sub or two will make a full range speaker seem to sound much better (magic!), and the low frequency "hall sound" on live recordings helps things sound like reality. Subs "charge the room" which, if you get used to them and simply turn them (or it) off for a bit you immediately notice the life force going out the window, so to speak.
