good quality equipment doesn't have a loudness control
The quality of the system is not the main factor in whether you have (or use) a loudness control.
The loudness control is to compensate for the ear frequency response versus sound intensity, not to correct for equipment deficiencies.
If you look at the updated Fletcher-Munson
loudness curves below, you see that the ear is relatively more sensitive to bass frequencies at a high volume as compared to a low.
To compensate for this, the loudness control adds bass as you lower the volume, so that the bass sounds relatively the same compared to the higher frequencies.
In other words it tries to make the lower volume curves have a shape similar to the high volume curves.
I had an old Eico amp that had both a loudness control and a volume control.
You calibrated it in your system by adjusting the volume to the highest level you listen to with the loudness turned to the maximum (where the loudness control response was basically flat).
Then you used the loudness control to adjust the volume.
That would seem to be the proper way to use a loudness control, if you have one.