An "equalizer" like for stereo amps sounds like the tool for flattening the curve. There is likely to be an explanation about how that is done somewhere.
Later:
Huh! So it looks like you just mix the weighted outputs of a low pass, band pass, and high pass filter together which is the way I imagined it but somehow seem not quite it. There might be some interesting wiggles in the frequency response curve that can be "tuned" out.
More study.