The “loudness” button was meant to compensate for perceived bass loss at lower volumes. Google “Fletcher - Munson curve” for information.
Note that as volume increases, the Fletcher - Munson effect decreases. In effect there is less need for bass compensation as volume increases.
At some volume level in any room will require no compensation. After you reach that point, activating a loudness button will artificially accent bass.
So the effect is not linear. I know of no “loudness” compensation button on any stereo system that took this non-linear behavior into consideration. When you activated the compensation you got the same level of bass boost regardless of where the volume was set.