A brief illustration and discussion of the equal loudness curves first developed through the work of Fletcher and Munson can be found here . Their work was refined by Robinson and Dadson with better equipment in the 1950s and research still continues in the matter.

It's unlikely that a simple on/off loudness button would provide a very sophisticated compensation. It might be worthwhile to use it when listening at a very low background music level.

-----------------------------------

Enjoy the music, not the equipment.