For those who have the time and aptitude for it, there is some excellent information to be found at:

http://www.harman.com/wp/index.jsp?articleId=120

One of the papers referenced at that link is:

http://www.harman.com/wp/pdf/multsubs.pdf

For those that want the conclusions first, in the room used for testing:

How many subwoofers are enough? – Four subwoofers are enough to get the best results of any configuration tried. Two subwoofers are very nearly as good and have very good low frequency support as well.

What is the optimal placement? – If cost and aesthetics are considered, subwoofers at 2 wall midpoints are preferred.

Approaching another aspect of ‘subwoofering:’

http://www.harman.com/wp/pdf/Loudspeakers&RoomsPt3.pdf

I found, by my personal mix of rudimentary objective measurement and subjective listing that:

Two subs are better than one, especially in real world, far-from-ideal listening environments. Depending on the type of music being played back, operating two subs in stereo instead of dual mono may have some benefit, and would likely have that benefit only for certain ‘live’ recordings where stereo bass was recorded in the first place and preserved through to the final playback medium.

There is no substitute for learning some basic subwoofer placement principles beyond what is found in most owner’s manuals and taking a good deal of time to experiment with subwoofer placement in your particular environment. The subs' output level must also be very carefully tweeked. There is no universal, ideal, single solution for every environment.

In my listening environment, with a convoluted arrangement of walls and spaces adjacent to the media room, I have found the following helpful:

A slightly asymmetrical placement of two subs means both subs are not exciting exactly the same nodes in the same places and helps reduce the places were bass is either boosted or suppressed by multiple standing waves affecting the same point within the listening area. (I have worked for the best listening overall in a listening area, not just a single ‘prime location.’ If using parametric EQ, this would mean using multiple sampling locations.)

A change in the position of a subwoofer by as little as three inches can produce a noticeable change in listening results.

I ‘de-couple’ the subwoofer cabinets from my ‘flexible’ flooring with some ‘cushioning.’ (Contrary to what much product ‘hype’ says, most spikes actually increase the physical coupling of a loudspeaker’s cabinet vibrations to the flooring. It’s physics.) This helps the ‘sounding board’ flooring in my situation to be more of an acoustical absorber and less of an acoustical generator.

I've experienced no problem with mixing .1 channel bass with the redirected stereo/mono bass from other channels via an Outlaw ICBM. My adjustment goal was that musical instruments with solid bass content would sound as they should - the low fundamental frequencies coming from the subs blending properly with the remainder of that instrument’s sound that come from the other loudspeakers in such a way that the original character of the instrument is faithfully reproduced. Once my painstaking pursuit of the correct music sound was as complete as I could make it, I found the reproduction of DVD movie material fell right into place without modification. I make no changes to the subwoofer setup as I switch between music and theatrical release.

Be patient, don’t be afraid to try different (non-destructive!) things.


[This message has been edited by bestbang4thebuck (edited August 24, 2004).]