To my knowledge, it boosts bass at low volume levels, gradually reducing the boost as volume increases.

That is correct. Its purpose is to compensate for the perceived non-linearity of the frequency response of an audio signal. As you turn up the volume, the bass seems to get louder more quickly than the treble does.

There is apparently a curve of which frequencies it boosts and when but nobody has ever published it AFAIK.

The empeg guys just implemented whatever the DSP programming manual recommended. They took it a step further than most car stereos do, by making it a scale instead of just on/off.

I would like to have another parameter in addition to the scale: The upper cutoff volume. As it stands right now, there's still a little bass boost even at maximum volume. I would like to adjust that point downwards so that there is zero bass boost at about -7db. How about it empgeg guys?