Measuring speaker distortion is not a trivial matter, since there are questions of what frequency and power levels to use. Richard Heyser was one of the first (maybe the first) to include comprehensive distortion analysis in his reviews. I've copied a section of a review he did so you can get a feel of how it could be done (or at least how he did it). There is more, with graphs, and this is only a small excerpt. But you can get the idea.
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Measured harmonic distortion for the frequencies of 41.2Hz, 110Hz, and 262Hz... correspond to the musical tones of E1, A2, and middle C. I chose middle C rather than A4 (440Hz) because the actual acoustic crossover from woofer to mid-range is slightly above 250Hz, and I wanted to measure the distortion for the same driver at all frequencies.
Low bass harmonic distortion progresses smoothly from a few tenths of a percent at 100mW to near 10% at 60 average watts, with second harmonic slightly above third harmonic throughout the whole range.
Mid-bass harmonic distortion at 110Hz does not look at all like the low-bass distortion. Mid-bass harmonic level stays essentially uniform and of low level throughout the entire drive range, as the woofer really likes the acoustic load in this important frequency range, and pumps out acoustic power with little distortion.
Up near the crossover the harmonic distortion rises uniformly with drive level, although its total level is very low even at a thundering 100+ dB SPL.
Intermodulation of middle C and E1, when both are mixed in equal proportions, is impressively low. Music played at a 10 watt average level is reproduced at SPL which many other fine loudspeaker systems simply cannot reproduce, yet IM remains below 4%. The nature of this IM is principally amplitude modulation of middle C by the lower tones up to about 50 average watts; at 100 watts (ear protection level) the IM measures 12.88% and has picked up phase modulation of 6 degrees peak to peak on middle C, in addition to about 8% peak to peak amplitude modulation.