Check in your local library for audio technical books.I had one with a picture of the B&W

Re % and db,no because db are log scale.
"Distortion is higher at 20 Hz than at 30 Hz even though the displacement is the same. This is a curious result. The fundamental drops 40log(30/20) = 7 dB, as expected. The 2nd and 3rd harmonics, though, remain at nearly the same sound level, as the voice coil swings through the same range for each of the excursion dependent non-linear parameters. Thus the distortion percentage increases"

-I dont think so-

• Linkwitz lab