Manufacturers, writers and salespeople toss around all kinds of numbers and terminology that can be very confusing and misleading. "This $300 shoebox sized sub is flat to 20Hz". Right, in your dreams . . . How is that cheap, tiny box and driver going to reproduce a 56 foot wavelength with enough power to be heard? It will not to it. Good bass reproduction requires moving a lot of air and playback at realistic volumes. Remember the rule of needing to move four times the air to go down one octave. Example You have a pair of good quality tower speakers with 10" woofers that produce good bass down to around 40Hz. The salesman is telling you that his little subwoofer with a single 10" woofer will extend your system down to 20Hz. If you've been paying attention, you know that his woofer will have to move eight times as much air as each of your 10" woofers, not likely. Adding that subwoofer to your system might give you more apparent bass energy, and in fact may help a little with movie special effects, but it is unlikely to extend bass response significantly.