Now that Bluesky has taken his obligatory pot shot, let's get back to bass management (I think this will be a long post).
Understand, at the start, we very rarely see "bass management" in use in large venues. It was developed specifically for use in the residential environment, or, let's say "small room acoustics". Low frequency modal issues in large rooms aren't the issue they are in smaller spaces.
In smaller rooms (whether you have two fronts, or three), one of the objectives is to create a wide and deep, 3-dimensional, if you will, sound stage. Problematic in residential spaces is it is very difficult to get any speaker sufficiently far away from any room boundary to not have an adverse impact from SBIR (Speaker Boundary Interference Response). To achieve an excellent sound stage, speakers are moved, twisted, raised, and/or tilted until that sound stage is as good as it can be. To further craft the sound stage, various forms of acoustic treatments are also common. This width and depth of the sound stage affected by the mid to high frequency range ... not by low frequencies.
So now comes the low frequency response (in the seating locations) problem. Ignoring for a moment that most "full range" speakers are not, the challenge is to create smooth bass response in the seating location(s). In the low frequency domain, modal response is the killer. Modal response issues can be addressed by careful subwoofer placement (that crawling on the floor thing), the use of some number of subwoofers (that number being between 4 and 5000) ala the Toole, Olive, Welti, Devantier method, or Gerry Lemay's "virtual" subwoofer method. Another, very rarely used method is to deliberately create constructive and destructive interference between multiple subwoofers to create smooth bass response (not commonly used because it is tricky and very, very time consuming). None-the-less, modal interaction within the seating area is highly dependent upon subwoofer positioning (moving the seats is often not an option because that sound stage gets corrupted in the process).
In the end, it is more often than not found that the best positions for the main speakers to craft an excellent sound stage are exactly the wrong positions for low frequency drivers to provide a manageable low frequency response at the seating locations.
Effectively, bass management allows low frequency content to be removed from the L/C/R (and surround) speakers and be redirected to one, or more, subwoofers. Once physically divorced from the main speaker, the low frequency drivers can be moved to those positions which provide a more optimal response at the seating locations.
One of the more common arguments against bass management have included, "I can hear the sub (localize it) and the positional disconnect from the mains is annoying." The fact of the matter is, you cannot localize low frequency sources (say 80Hz and below since that is a common crossover point). It is not the low frequencies which are being localized. Subwoofers can easily create higher frequency artifacts of their efforts to create the low frequency sounds. This includes mechanical sounds, the cone slapping the air, and so forth. These sounds can be easily eliminated. A further cause of distress can be linked directly to poor integration of the sub(s) to the mains. This integration cannot be done with just an SPL meter. If one "hears" the subwoofer(s), the calibration and integration is poor at best. In a properly calibrated room, the fact the surround speaker's low frequency content has been relocated to multiple subs is not going to be detected by the listener.
Now there are risks which can be introduced into the listening environment once you have excellent low frequency performance to 20Hz or below. These risks go back to the recording and production chain. Let me give an example: A Grammy winning recording artist produced a track where the 35Hz content would take your head off. It was very loud and horribly distracting (on the bright side most car stereos and all iPods couldn't reproduce 35Hz on a good day). This poor mix occurred because the mix engineer was sitting very close to the peak of the 35Hz null of the mix room! [EDIT: peak of the null ... near where the null was almost at its deepest point). There are other examples such as hearing the train passing through the London Tube below the recording studio ... the playback monitoring system wasn't capable of reproducing that particular low frequency range. It was on the release, most never heard it, but those with capable systems did, and, correctly, gave it a poor review. On the other hand, the Telarc 1812 (re-released as an SACD #60646) did have very deliberate way below 20Hz content. The Telarc Holst Planets with Fredrick Fennell (re-released as SACD) does as well. (These were both recorded on our Soundstream system and originally released for vinyl.) In any event, once you get a system which can accurately reproduce below 20Hz content, be prepared for some surprises (or disappointments as the case may be).
Dennis Erskine CFI, CFII, MEI
Subject Matter Expert
Certified Home Theater Designer
CEDIA Board of Directors