I've been following this thread and have decided to weigh in and I acknowledge that all of my comments are simply my observations and opinions. I am not as interested or competent in the technical aspects of audio as @erik_squires, @millercarbon and others who frequently post. I am, however, an avid music listener and my wife and I both play musical instruments and frequently attend classical and jazz concerts. Our audio system is the vehicle for our music listening.
1. I agree with @rauliruegas that for some listeners adding one or more subs that generate significant distortion may be worse than not having a sub. I say "for some listeners" because if someone is adding one or more subs in an attempt to create the sound of a live rock concert, adding subs that generate distortion is overwhelmed by the distortion already in the music, created by the electronics of the performing artists - think Led Zeppelin, Deep Purple, The Who, etc. Having subs that produce generous amounts of bass will help get one closer to the high volumes and bass of such groups.
2. I also agree with another contributor who noted that adding one or more subs may be less important for those who listen to acoustic music - classical and jazz, folk, etc. While much classical and most jazz do have significant amounts of bass, the acoustic bass is typically part of a large orchestra in classical and is typically part of a rhythm section in jazz. For such music, and particularly for chamber music, folk and some bluegrass, the bass frequencies are a less significant part of the overall performance than in rock music, although much symphonic music has a significant amount of percussion and there are organ pieces that will challenge the best audio systems.
3. I will not disagree with the benefits of a well integrated subs or subs. They can improve not only the bass response of an audio system, but also improve the soundstage and the response of the drivers in the main speakers. This leads to another point.
4. What has not been mentioned in the prior posts is the importance of having sufficient and high quality amplification for the main speakers of a system. To provide low distortion and lifelike transient response in something like a large symphonic work, the amplifier(s) need to match the requirements of the speakers. As an example, our primary speakers are a pair of Aerial Acoustics Model 8b's. These are modestly sized, three-way floor standing speakers that are relatively inefficient (86 dB sensitivity), with published frequency response of 28 - 22k Hz +/- 2 dB. They are 4 ohm speakers that drop to almost 2 ohms impedance at certain frequencies, a challenge for most amps. Fortunately, our then dealer paired them with a Proceed (Levinson design) HPA-2 amp with specs of 250 w/ch at 8 ohms, 500 w/ch at 4 ohms. The amps have never clipped that I could hear in the years we've owned them and provide high current that drives the 10.7" woofers in the speakers with aplomb. I mention all this because a few years ago I auditioned in home a high-end Luxman piece as a possible replacement for our amp. The Luxman was rated at 150 watts/ch at 8 ohms so I figured we were fine. Wrong - the clipping was very audible and obvious at high volume transients while listening to a symphony. If I had our M&K sub integrated into our two-channel system the Luxman amp would have been splendid for at lower volumes it produced lovely, neutral and highly detailed sound.
5. I have benefited from tips provided by @millercarbon in several of his posts and in direct communications with him. His recommendations for acoustic treatments noticeably improved the music's clarity and soundstage in our listening room. I, however, share the view that while subs can be very helpful, they are not necessarily the end all. And the DBA, multi-sub approach advocated by @millercarbon is not feasible in many multi-purpose listening rooms even if one uses wireless subs. I cannot dispute the DBA may sound "better" whatever that means, but for us integrating even one M&K sub into our system was a challenge and I don't want to have to adjust the bass volume as different albums are played. I want to spend my time listening to the music. So our sub is used for home theater but not for music and we are content with the reproduction provided by our speakers and amp with both analog and digital sources.
