Dual vs single sub

phusis you are right. A point source system can be wonderful with two subs set up correctly. A line source requires four subs in most rooms (wall up to 16 feet). Now I am going to make some people mad. Distributing the subs through out the room makes no sense. Once the subs are more than 5 to 10 feet apart depending on cross over point they act as two completely separate drivers with 2 separate interference patterns in the room. At 10 feet you would have to cross over below 40 Hz. Systems benefit greatly from high pass filters. It relieves the main amp from having to produce low frequency transients which require a lot of power. They also eliminate Doppler distortion from the satellite low frequency drivers cleaning them up. You get more improvement the higher you cross (but not too high). But with distributed woofers you can't cross above 80 Hz or you screw up the image (some would say lower). If you want the most benefit from multiple woofers, the lowest distortion from your satellites,
A great image and absolute thundering bass with the lowest room interference, set up the four woofers along one wall right up against it symmetrically in regards to the satellites right and left channels. The far woofers should go right in the corners, the near just to the inside of the satellites. Cross over between 100 and 130 Hz 12 to 24 db/octave and use high pass filters. Don't believe me. Try it for yourself if you have the equipment. I do have the equipment and I have been using multiple subs since 1979 and have set them up in every configuration you can think of using every conceivable cross over. Experience is always the best teacher. Aside from listening to music this is where the fun is in this hobby, playing around with your system. Once you have the equipment you can play around with the configuration room constraints allowing. As long as you are a good listener you are in business. Back to Stravinsky! 

Oh, by the way I use dipole speakers with sealed subs and have absolutely no problem matching them up

I have made dipole subs using 6 drivers aside. They do not work. Even with huge amounts of power and room control you can not get 20 Hz out of them effectively. IMH experience the best performance comes from sealed, enclosed 12" subs Q @ 7 with Room control and a high powered class AB amp damping > 500, the higher the better. The only difficult variable is the enclosure itself which ideally should be infinitely stiff and heavy. 

For all you people planning subs on a limited budget just buy the first one and add another when you can. For point source speakers 2 subs will get you 90% there. For you panel jockeys, Magnapans and electrostats you are going to have to shoot for 4. 

Oh, by the way Duke, I am not trying to sell anything.

Audiokinesis, It always helps to know what ideal is to make the best compromise in any given situation. 4 subs at 4 foot intervals with the outer two in corners does not approximate a linear array. It is most definitely a linear array up to 125 Hz or so. If you review the acoustical properties of linear arrays you will note that they throw very little acoustic energy off their ends almost totally killing any refection off the side walls of the room. No other array does this in any dimension.  The best you can hope to do in any enclosed space approximating a livable room is to control primary reflections and since they have the most energy that represents 90% of the battle. Of course most people are stuck with a rear wall. My point is that all the room treatment in the world will not stop that reflection in the subwoofer region. The wavelengths are simply too long. Putting subs all over the room simply creates more primary reflections. For most people with normal rooms digital room control is the only way they stand a chance of getting flat bass at their listening position. The horizontal linear array minimizes the problem to a degree no other set up can and it increases efficiency at least 6 db requiring 1/4th the amplifier power which is critical with room control that frequently has to correct troughs of up to 10 db and more. This is why we need so much power when room correction is used particularly with small sealed woofers that have to force response below 40 Hz. The only other ways to achieve flat response at the listening position are to move the listening position to a point in the room where the bass is satisfactory or move the subwoofers around in the room in a trial and error way to accomplish same. Good luck. 

Audiokinesis, If you create a linear array very little energy disperses off the top and bottom or in this case to the side walls minimizing that interaction. Because the woofers are up against the front wall there is only the primary wave and no separate primary reflection off the front wall. So, you have eliminated three primary reflections. The reflections from the floor and ceiling are scattered in time. The reflection that will cause the most trouble is the one from the rear wall. The only thing that really works here is just get rid of the back wall. I am fortunate in that I designed my media room without a back wall. It just opens up into the rest of the house scattering the rear primary reflection. The end result is that the bass intensity is even throughout the room and much less equalization is required. If you can't get rid of the back wall you will be stuck with a degree of "comb filtering." I am not sure if spreading other sub woofers around the room would minimize this or just create more complex patterns. In reality it only really matters at the listening position.
The best bass is bass you can feel.

I think Noble is on the right path here. Look at the difference between the radiation patterns of point source and linear arrays. The trick is in creating a linear array woofer. If two drivers are closer than 1/2 the wavelength of the highest frequency they are to reproduce they will act acoustically as one driver. The wavelength of 100 Hz is around 10 feet. So, if you space subwoofer drivers 5 feet apart they they will act as one driver up to 100 Hz. Say you have a 15 foot wall. Space 4 subwoofers at 5 foot intervals along that wall the outside two being right in the corners. The other two in the center should be pointed at each other with the drivers as close to the wall as you can get them. What you have created is a horizontal linear array subwoofer which minimizes room modes and reflections and takes full advantage of intersection gain. 12" drivers are way more than enough. Many will do fine with 10" drivers.