One thing to keep in mind is there are two types of subwoofers. One is made to gel with your system and the other is to go low.
In most rooms once you work with 250hz and below you will have two different sets of pressure zones that react differently depending on your room's material and construction. If you have basic stud and drywall construction with wood floor joist you will get more musical tones to work with. If you have plaster walls or double drywall or quiet rock and a basement, you might end up needing two sub designs.
I have found that almost all newer high end audio main speaker designs end up wanting a sub to complete the range.
Michael Green