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I think a better excuse for buying two subwoofers is for stereo imaging reasons. Most folks place their speakers too close to each other to appreciate the effects of stereo imaging. Maybe this comes from all the near-field listening we do with our comp speakers. Anyway, subs can get away with going solo because of the non-directional nature of low frequency sound waves. Still, true stereo effect is two distinct channels being played through a right and left speaker. Does this not make more sense than the "leveling out irregularities" theory? I've never heard that one before...

A couple of sub tips from a hardened audiophile:

1) Corner placement usually works best to minimize standing waves and boominess
2) If you can hear that the bass is coming from the sub, then you need to turn down its volume. Most people have the volume set way too high. Turn up the volume until you can begin localize then sound, then turn it down until you just can't
3) Two subs can indeed smooth out room irregularities, but ought not be needed unless you have a troublesome room.

The idea of leveling irregularities with two subwoofers is right on. Low frequency sound waves are much larger than mid-range and high-frequency sound waves, with the lowest frequencies within our hearing range measuring up to 15 feet! That means you could possibly be sitting in a bass "hole" where the sound wave completely passes over you. Adding a second subwoofer will minimize the bass blind-spots and help even out the sound.

As far as imaging with your subwoofers, it's not something you want to do. You shouldn't be able to tell where the really low notes are coming from. The whole point of imaging is that your speakers create the sensation of a sound embodying a distinct point in space between your speakers. Very good speakers will do this like magic, as will really bad subwoofers.

The real problem is that so many people have been scamed by Bose (and their many, many followers) that tiny speakers can give a good home theater sound. While aesthetically pleasing, those speakers rely so much on the sub to create full range sound that the whole imaging issue is real for them.

With good full range speakers (which are necessarily larger - 8" Drivers at a minimum in my opinion), the sub should really be adding as much "feel" as anything. A little meat on the bass guitar, the rumble of an explosion or a truck going by. Definitely agree that most folks have the sub turned up way to high (usually to counter the limitations of their main speakers).

On placement, I'm a fan of corner placement (effectively horn loading the speaker) which seems to raise fewer resonance issues. Some rooms, however just don't play nice with subs and may actually be better without one, unless you're willing to invest in sound treatment.

I wanted to correct a few mistakes posted above.

1. Whether listeners can really appreciate significant differences between stereo bass and mono bass reproduced from two subwoofers is controversial. Setting the crossover sufficiently low with a sufficiently high slope will help reduce the localizability of the subwoofer.

2. Corner loading will maximize standing waves and resonances, as well as bass output from the subwoofer. However, if room dimensions are not multiples of each other, the frequencies of the standing waves may be sufficiently be spread out to have reasonably smooth bass. This will vary significantly from room to room, however.

3. 20 Hz bass tones have a wavelength of 56.5 feet (the speed of sound is approximately 1165 feet/second, though this varies on altitude, temperature, and humidity).

4. Bass nulls are due to cancellations from standing waves, not "the sound wave completely pass[ing] you over." A second subwoofer may not excite the all of the same standing waves as the first, but it's possible to place the second subwoofer to excite at least some of them.

A great resource for beginners are the white papers on the Harman website, particularly the "Getting the Bass Right" one.