It is also possible that Messers Fletcher and Munson may have some useful input on this.
Human hearing is not linear. Usually, that means that something with twice the acoustic power is perceived as only a little louder, while something with ten times the acoustic power is perceived as only twice as loud. In this case, it's a different kind of non-linearity I'm going for. We don't hear all frequencies the same. Specifically, when all are at the same acoustic power, we hear midrange best, treble less well, and bass not nearly as well. To complicate matters further, those relationships change with volume. At low levels, we don't hear bass well at all, but, as levels increase, the bass increases faster. Fletcher and Munson demonstrated this back in the 1930s. So, if we adjust our tone controls to get a good sound at low volume, then turn it up, we are necessarily changing the perceived EQ. Essentialy, we're turning up the bass more than anything else. This can be the reason a setup can sound anemic at low volume, then really bloom when turned up.
I don't know if that's what's happening here, but it's likely part of it.
https://en.wikipedia.org/wiki/Equal-loudness_contour
