@jrpnde , nice article and very correct. A sub not only has to match the mains in frequency response but also in time. This is the problem with DBA's. They do a great job of evening out the bass response throughout the room. But, they are not necessarily and probably not matched in time with the main speakers. You have to be in phase and in time with the main speakers or you have essentially an echo. Bass transients like bass drums lose their impact. The buzz in bass strings disappears. The only way you can do this easily is with digital signal processing. The Anthem system was mentioned above. MiniDSP makes the least expensive unit that I know of. There is Trinnov, DEQX and Lyngdorf. People who are digiphobes need not apply and more than likely would be better off without subwoofers. You can not recreate a live performance without subwoofers. To do it right takes much more driver surface area than you would think. Residential rooms stifle bass. Add a processor to this and you have considerable expense. If you can not do it right you are more than likely better off from a sonic perspective not to do it at all. You can kiss the live performance goodbye but this is not what most people want anyway. They want a more polite system that is very detailed. They rarely listen over 85 dB and bass loses it's energy at this volume subwoofers or no.
Another important aspect of subs is that they take the load off the main speakers which can make serious improvements with their performance in terms of distortion and headroom. Good subs do not low pass at 40 Hz. They are used in conjunction with a high pass filter on the mains and cross between 80 and 120 Hz then time corrected usually by delaying the signal getting to the main speaker so the group delays match at the listening position. You have no idea what is happening unless you measure it. If you think you can do it by ear, good luck to you. Getting it right would be like winning the lottery. Like everything else in this life there are no cheap easy solutions only people who want to sell cheap easy solutions.
Sorry for preaching.