That, to me, is a very strange study to be using pure tones of 'distortion' during music. Contrary to claims by the study, that is not how speakers behave. It doesn't prove much because the tones used do not relate to the masker, which in that case was music which is complex content. That says nothing about the audibility of harmonic distortion, and I am not sure what is says about masking either.
By far, the best piece of research done on the audibility of distortion in bass frequencies was done by Louis Fielder and Eric Benjamin:
Fielder, Louis D., and Eric M. Benjamin. "Subwoofer performance for accurate reproduction of music." Journal of the Audio Engineering Society 36.6 (1988): 443-456.
In fact, it is pretty much the only serious study that has been conducted on this subject.