This blog post outlines some performance criteria for achieving low distortion bass in your listening room. These metrics are my own criteria that I've developed and represents a good baseline for audiophile two-channel music systems. I will also show practical design approaches to achieving those targets. These guidelines can certainly be used for Home Theater applications as well.
My performance targets are:
- Total Harmonic Distortion (THD) percentages below 0.50%
- Listening levels of up to 100dB SPL at the listening chair
I encourage you to perform your own acoustical measurements on your own system as a comparison against mine. This will ultimately help you decide if your system needs upgrading.
Test Setup
My measurements were conducted using the ground plane method. This places the speaker and microphone on the floor at 1 meter apart. This method helps minimize the effects of the room's acoustics on the measurements.
Measurement Method
I conducted frequency response sweep tests using REW with gradually increasing output until I reached around 10% distortion. I increased each subsequent measurement by 5dB increments and charted the resulting distortion at 40Hz. This provided a good snapshot of the distortion profile against the overall output SPL. Generally a speaker will hover around a certain distortion level until a certain SPL is reached where it will quickly escalate into the double digits percentage. However you'll see that not all drivers are created equal.
Test Sequence #1 --- SLS Self-Powered 10" Subwoofer
The chart below shows the distortion profile as SPL output increases. As you can see distortion never goes below 1% and quickly rises starting at 90dB SPL at 1 meter. So the conclusion based on my criteria mentioned earlier is that this sub is simply not suitable for audiophile applications since distortion never gets down to 0.50% and certainly doesn't have the output capability.
Test Sequence #2 --- Fostex FW208HS in 50 Liter Onken Enclosure
This test uses my own cabinet design and driver for this test. The plans for this cabinet using this driver is available on my site.
Below is the distortion profile over SPL output for comparison. As you can see this Fostex woofer has very low distortion within it's SPL capability. Maximum output is pegged at just over 100dB SPL before distortion crosses the 0.50% THD threshold. This speaker meets my performance criteria by offering less than 0.50% distortion and achieving 100dB SPL at the listening chair if a stereo pair of speakers are used and the listening distance is 2 meters or less. This equates to a small listening room. In other words this driver would not be suitable in larger listening spaces due to the limited SPL output.
Test Sequence #3 --- Dayton Audio RSS390HF-4 15" Reference HF Subwoofer
Below is the distortion profile over SPL for the Dayton 15" subwoofer in a sealed enclosure. As you can see this subwoofer just barely achieves the distortion threshold of 0.50%. Distortion rises past 0.50% at 100dB. However the story is not over if we look at using multiple 15" subwoofers which provides a nice segue to our next test...
Test Sequence #4 --- Qty: 2 Dayton Audio RSS390HF-4 15" Reference HF Subwoofer
Below is the distortion profile over SPL for quantity 2 Dayton 15" Subwoofers. As you can see distortion drops to well under 0.50% with a maximum output of 117dB SPL. This equates to 108 dB at 3 meter listening distance and 106dB at a 4 meter listening distance. This means you would have at least 6dB headroom even in a very large listening room. The conclusion here is that dual Dayton 15" subwoofers would be very suitable for medium to large listening spaces. 
Conclusion
Testing has shown that the Fostex FW208HS is suitable for small sized listening rooms. From testing the Dayton 15" subwoofer is was shown that dual subwoofers reduced %THD by nearly 50%. This brings strong design rationale for multiple subwoofers. It would be fair to assume that increasing the number of driver to four 15" subwoofers would reduce %THD by even more. This is all in the context of normal listening levels (95-100dB at the listening chair). Looking back at the Fostex FW208HS test results we can also reasonable assume that distortion would also drop when using them in a stereo pair similar to what we observed with the dual dayton arrangement. Since I used the ground plane method my test results do not account for the room's acoustics which is also a major contributor for the overal sound quality.
Footnote 1
After some discussion on the forums I want to clarify what this method is and what it isn’t. This method outlines a practical approach to optimizing a bass solution for two channel audiophile music systems, within the context of the DIY enthusiast. This is not a new standard that could be published in a speaker’s specification. It is expected that this can be used as a tool to navigate improvements to the system including upstream components such as amplifiers. In fact I had to swap out multiple amplifiers to achieve distortion below 0.50%. This is exactly the type of process that I’m proposing the enthusiast try!
The 40Hz selection is based on observing all the data once it’s collected and deciding on what correlates best based on what seems linear in terms of increasing distortion with output. Additionally, you should focus on a part of the bandwidth that is above the tuning frequency of a bass reflex cabinet.