The main problem with subwoofers is frequency doubling, especially at 60Hz and above. A good test of your sub for this is to obtain a sinewave sweep generator and sweep the sound from 15Hz up. common troubles usually begin at 50 Hz where as the sweep up occurs most people will begin hearing actual sound at 30 Hz but when the doubling frequency is reached the sound gets not only louder but a new second harmonic pitch is detected. There are many reasons why this happens, including the room shape and size. Really good subwoofers will not double until you get into the upper range of the system, often around 100 Hz. It is better to have large mains that can extend to a lower frequency than the upper range of the sub. Then select a crossover frequency for the sub's upper limit at below the doubling frequency. Generally crossover networks for mains do not address a low cutoff so here you just use a main with the largest low end air movers you can afford. Now back to the subs and how to improve their sound quality.
Lets assume that you do not have a choice in the room size and design but you should consider that the room will have a lower limit to cram the wavelength of a low frequency "sound" into it before the pressure wave hits a reflection wall and gets doubled back on itself causing pressure wave collisions. These often are said to "muddy up" the low end. Adding multiple subs in the room will add to the issue of pressure wave collisions and increase the muddiness or reduce low frequency purity. It has long been the opinion that in smaller rooms, use one sub (location) to avoid this. So what is considered large and small room size with respect to low frequency components. I'm going from memory on this but a theater room 30 ft long by 20 ft wide and 10 ft floor to ceiling will fit the wavelength of 30Hz before pressure wave collision. Add a second sub in a different location and that simple formula goes out the window. There will be collisions all over the room. Some will be additive and some subtractive to the punch of the low end. If the room in double that size you can fit a 20 Hz pressure wave before collisions. double the size again and we allow an even lower frequency. So, soon we approach the size room of a cinema theater. Now a single sub location starts to achieve directionality. Here, location of sub(s) becomes more important and the AES has recommendations for placement to eliminate directionality of the pressure wave for the .1 channel. Note some experimentation for .2 channels where the subs are expected to have directionality but there is very little distribution media cut for a 5.2 or even 5.4. You will find these configurations present in large live stage concerts where the pressure wave from the subs are designed with directivity in mind.
OK, so you want very pure and clean pressure wave in your 20x20x10 Home theater room, what next? a single 12" or 2 x 8" ? The 2 x 8" subs transducers in a single enclosure will have about 400" sq in of air moving surface. a single 12" is 452" so a single 12" would have more push power per watt than twin 8" My sub has 2 x 10" transducers which I built myself from Sony Xplodes with two of them in series for 8 ohms. These have about 630 sq inches of air handling vs the 452sq in of the 12" and 706 sq in. of the single 15". But a single 15" with adequate magnet at 8 ohms cost double the twin 10 in subs. MY design was to maximize performance against cost and the twin 10" did that.
To increase further, the sound performance of the sub, I went with a high power amp. My twin 10" xplodes are driven with a Carver ( relatively low cost) 2000w subwoofer amp. I moved up from a 400 watt amp to a 1000w amp and let me tell you the low end punch and smoothness is equal to some of the finest cinemas around with 2Kw. It isn't loud but a balanced pure low end punch. We had a 12" Veledyne here and my system way out performed the Veledyne as the Veledyne had a 70Hz harmonic we could not eliminate. (in other words, the 35Hz punch produced a phantom 70Hz sound that was not part of the program!) My system cost less than a third of the Veledyne. I was ready to buy that box because it had what mine didn't which was furniture appearance! Imagine my disappointment when it didn't perform as well as my home made box constructed from car audio components.
crossover balance: I said the mains should have the largest low end transducer you can afford and let it extend into the cross point of the sub's upper range. I have two JBL studio monitors which have a 15" woofer 20 lb magnet and have a low range of about 40Hz before fast roll off. These are not drives as sub woofers but simply as woofers and passively connected to the front left and right channel's crossover network.
About base shakers: I have my main seating couch suspended acoustically and the frame fitted with a single Butt Kicker transducer wired to a stereo 2Kw amp that is bridged. Let's just say that thing kicks butt! But after I had my open heart surgery, I had to disable it because of the broken ribs and sternum I was in too much pain to be sitting on a back slapper. The healing took over a year. Funny, I don't miss it as I haven't connected it since before my surgery. When I bought it I tried the base shakers and IMO, don't waste your money. They are a toy compared to the Butt Kicker. But if you can afford it and really want the Rolls Royce of these, get a D-Box system. I still have dreams of owning one of those some day, especially for games. I was saving up for a DBox when 3D came out so I went that direction instead. Some day I'll get a DBox, when the wife isn't looking
PS- when I was in my 20's I designed the delay lines for an engineer friend of mine for the Watkins Glen Grateful Dead concert. I know a little bit about subwoofer design.
