The CEA-2010 thresholds for distortion are too high in many's opinion (including mine). This chart is from the REW documentation.
IMO, the REW Signal Generator CEA-2010 burst tone is a great tool for all to use. It is much safer than a constant sine wave, so no worries about cooking voice coils. If something sounds terribly bad, it can be stopped after just one cycle. I doubt it could damage subwoofer with a limiter, even if one tried (don't try). You don't even need a mic to use this tool. It is great for chasing down sympathetic vibrations.
Noise detected this way can seem inaudible during most movie scenes. However, it is there and can muddy up the audio. Reducing noise will increase dynamic range and improve your listening experience. It is your room's S/N ratio. The lower the noise floor, the less volume you'll need to enjoy content to its fullest. The lowered demand on your system also will lower distortion, especially the bass.
With a microphone, you can use the CEA-2010 burst tones and the RTA to evaluate distortion at specific frequencies. It also a good way to determine your own threshold for detecting distortion.
I recommend starting at 63Hz. At low volume levels THD should be below 1%. As you increase the tone will get louder. As the subwoofer begins to approach max output and distortion rises you should detect the tone itself has changing. You will be hearing the effects of the extra harmonics. You may hear other signs of the sub struggling (driver & port noise). This is where it is time to stop increasing the level.
I hope this all made sense.