mapman linked
https://en.wikipedia.org/wiki/Equal-loudness_contour You want a system with automatic ’loudness’ circuit. It is completely misnamed, it is a ’low level boost’ to boost bass progressively as volume lowers, (boosts highs also, less importantly). Maintaining bass at low volume will maintain involvement with the music, otherwise it becomes background noise.
To compensate ONLY when listening at low volume. Raise the volume, eliminate the boost, otherwise the bass is too prominent as well as the highs will get screechy.
As you raise the volume, you want the circuit to automatically/progressively reduce/remove the ’loudness’ boost, back to normal for normal or louder.
This unit has a built in automatic fletcher munson circuit, and remote volume, remote balance, I wouldn't live without it.
https://www.ebay.com/itm/Chase-Remote-Line-Controller-RLC-1/254708363331?hash=item3b4dcd3043:g:huIAA...Vintage receivers have ’loudness’ filters.