I'm fairly certain the origin of the decibel (or bel, of which it is a
tenth) is that it is the minimum change in level that the average ear can
determine in its most sensitive range.

Nah, the magnitude of a decibel is simply based on 1/10 of a log10 and, so,
is mathematically determined. However, you may be getting confused with your
story, as when calibrated for use as a noise measure, 0dB is set at a
standard threshold of human hearing level by convention.

Christian.