"Loudness' EQ (should have been named 'Low Volume Bass Boost') is important for low volume listening.
i.e. Maintaining Jazz Bass Player in low volume listening preserves Involvement. If bass not maintained, becomes background music, no real involvement.
Fletcher Munson documented ear's sensitivity variations as volume drops,
https://en.wikipedia.org/wiki/Equal-loudness_contourresulting primarily in a need to progressively boost bass as volume reduces.
'Loudness' circuits, (some manual, some automatic) were standard optional features 'in the day'.
My vintage McIntosh Tube Tuner/Preamp has separate optional Loudness filter. All my Vintage Preamps/Receivers had variations of 'Loudness'.
Many used it wrong, gave it a bad name, minimal 'pure' signal paths ... it has disappeared unfortunately IMO.
IF not properly set up, the progressive bass boost starts too soon, too much bass, and remains too much bass. When properly engaged, it definitely makes a beneficial difference.
This Chase Remote Line Controller, RLC-1 has automatic built in bass boost, progressively as you lower the volume. Like vintage equipment, many set it up poorly, and the bass boost begins too soon.
https://www.ebay.com/itm/Chase-Technologies-RLC-1-Remote-Line-Controller-RLC1-Upgrade-Any-Stereo-NOS...You start/leave it at it's default volume; adjust your preamp/integrated amp's volume for 'normal'. No bass boost. Then, use Chase unit for increased volume, no bass boost. reduce volume, no bass boost until you lower below 'normal', when it then automatically/progressively/properly boosts bass, maintaining involvement.