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Longer answer...
In each of these studies, you will see a large number of outliers. We expect about 10 people in this population to hear combined distortion at -69dB, but in fact we see 305 people who appear to have maxed out the test.
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Either an unusually large population have outlier-great hearing, or they have good analysis tools.
It's easy to discern -69dB by looking at the spectra:
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I can do no better than -51dB on this test, and it is a bit exhausting, boredom being part of it. Some of these help at higher volume, which is a bit self-limiting. Each person's ear will affect the result depending on the distortion added, Klippel offers just a few modes, each person will have their own combinations they are particularly sensitive to.
More interesting than ego-stoking or soul-bruising discussions of super hearing abilities...

Klippel allows the assessment of different types of distortion commonly found in speakers (see attached):
www.klippel.de/fileadmin/klippel/Files/Know_How/Literature/Papers/Klippel_Nonlinearity_Poster.pdf
Nonlinearities in suspension, motor, and inductance all contribute unique distortion. The suspension compliance leads to harmonic distortion, the Bl and L(x) nonlinearities show up as IMD-like distributions. Here are the spectra of each distortion type in the test at -39dB, the individual nonlinearities are each offset by 50dB and all is smoothed to aid the eye.
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You can see the simple harmonics and sensitivity to low frequency of the compliance nonlinearities, and the IMD-like distributions caused by the Bl and L(x) nonlinearities.
PKane's
DISTORT is also useful for simulating distortion, if you want to explore. I think the Klippel pdf linked above poster and the actual nonlinearities explained is the really interesting part though.