A couple of responses mention lower overtones. This is not correct. An overtone is a frequency above and some multiple of a fundamental frequency. Thus the first couple of overtones for the 42 Hz of an open E string on an acoustic bass are 84 and 168 Hz. There is no 21 Hz overtone.
http://en.wikipedia.org/wiki/Overtone
From demonstrations I've heard, the most important benefit of a subwoofer (or bi-amped full range speaker if the bass crossover is low enough) is improved clarity and definition in the upper-bass, lower-midrange. I think this results from removal of the lowest source frequencies (requiring the greatest energy) from an upper bass driver and the main amplifier. I've found this case no matter what type (frequency range) of music is played.
http://en.wikipedia.org/wiki/Overtone
From demonstrations I've heard, the most important benefit of a subwoofer (or bi-amped full range speaker if the bass crossover is low enough) is improved clarity and definition in the upper-bass, lower-midrange. I think this results from removal of the lowest source frequencies (requiring the greatest energy) from an upper bass driver and the main amplifier. I've found this case no matter what type (frequency range) of music is played.