Richard F. Lyon asked:
"I'm open to the idea that ....... some kinds of psychophysical experiments
give a CB that doesn't vary with level, though I
haven't really seen the evidence for it yet.
Maybe you can point us toward some."
The best "experiments" for your answers are ordinary speech and music.
1) The quality of speech perception does not deteriorate when sound
level increases from 50 to 80 dB.
2) The consonance vs. dissonance balance of a musical chord does not
change when sound level increases from 50 to 80 dB.
These observations tell us that spectral filtering, including band
filtering, in the auditory system is as good as unchanged in this
level range.