We visit many houses where the subwoofer has literally just been 'plonked' in the corner and turned on, resulting in a sound that is, more often than not, worse than if it hadn't been used at all.
Here is our guide as to how to integrate a subwoofer into a system, both for movies and for music. All these adjustments should be done both when initially purchased and then again a month or two later when run in.
Play something with a repetitive bass and adjust the phase switch, until the bass, at the listening position is at its loudest.
If the subwoofer has side firing drive units, Again play something with a repetitive bass and rotate the subwoofer until the bass, at the listening position is at its loudest.
Place in a hard (ie brick, stone etc) corner in the same plane as the speakers. Again play something with a repetitive bass and move the subwoofer towards/away from the walls, until the bass, at the listening position is at its loudest. Small movements can make big diferences.
This can best be done through listening to music that contains deep, natural bass (ie real instruments rather than synthesizers). The crossover needs to be adjusted ensuring the bass does not overlap with your speakers, causing a bass hump, or is set to low, causing a hole in the bass. Then adjust the subwoofer volume until it integrates with the loudspeaker volume and sounds totally natural and not overblown,
The end result should be not only deeper bass but also sweeter midrange and a better soundstage.
Hi-Level / Low level:
Some subwoofers come with a Hi-Level (speaker) input. If your amplifier is happy with this type of connection it is the best way to go for music, as the subwoofer is receiving the same signal as your speakers.
This is the dedicated low frequency channel on DVD/ Blu-Rays. Connect this to the subwoofer on your AV amplifier. Set the crossover point on your AV amplifier and if the subwoofer has a crossover control for LFE, put this control to full (the amp will limit the frequencies going to it). Adjust volume to suit. Most of us are a lot less critical of the bass in a movie, as opposed to music.
One versus two subwoofers:
In a nutshell, two smaller subwoofers sound better than one big one as the bass is much more evenly distributed. I would normally set both up as above and then turn the volume down on each unit by two or three 'notches'.