Everyone hide in a corner it's a "SETUP"!
Most people think that subwoofer setup is the easiest of all the speakers to accomplish when actually it’s exactly the opposite. Most full range speakers whether they are L,C,R’s or surrounds, box speakers or in-walls you get what you get. Nothing on the speaker itself is variable with a few exceptions. A few manufacturers offer high frequency attenuation switches with something like +/- 2db and even a smaller number of manufacturers will provide you with various plugs to tune the factory ports on the cabinet allowing you to “tighten” up or “open up” the low frequency response of the speaker. Subwoofers on the other hand, in particular active subs, give you control of the individual volume, crossover frequency, possibly subsonic filter, phase switch or ½ space, ¼ space and 1/8 space switches. Before we discuss how to set up your sub to work with your system there are several things we need to know and questions we need to ask ourselves. Some of the most common questions are listed below.
read entire article by Jason Levert