Most good, powered subwoofers come both with a continuously variable choice of crossover points from 50 to 150 Hz and with a volume control to adjust the sub's output to match the levels of your main speakers. Manipulating these controls together gives you a very wide range of choice and control over the amount and nature of the lows you experience. They want to be moved at least a bit, and the amount of air required to move them is considerable. This is possible, of course, only when you have a separate amp and preamp, or when an integrated amplifier or receiver has a special, volume-controlled pair of line outputs. The final advantage, which is more meaningful on music than with Home Theater material, is that a good sub can be connected to route only the frequencies above its crossover point to a systems's main speakers.
Many people report notably smoother sound from their main speakers when the lows go only to a sub.
The reason for this is probably lower intermodulation distortion in the midrange of a speaker when its cone is not called on to make long excursions for bass. It's doubtful whether the improvement has any meaning in Home Theater, but if you like to listen to complex orchestral music at lifelike volume levels, you may well feel the improvement has great value.