The "loudness" button on old equipment is actually a combined bass and treble boost, designed to the make the music sound louder at low listening levels by comp

The "loudness" button on old equipment is actually a combined bass and treble boost, designed to the make the music sound louder at low listening levels by compensating for the effects of the equal loudness contour. It's a great idea for anybody who cares about protecting their hearing. You can do even better using modern DSP by equalizing to a curve based on the difference between the equal loudness contour at your preferred listening level and the equal loudness contour several decibels louder.