In an ideal world one would want speakers as uniform in power response as possible, a load invariant amplifier, and a room treated to offer properly placed speakers minimal early reflection. Such a system will “open up” roughly in lockstep with the Equal Loudness curve, aka Fletcher-Munson or ISO. If you apply corrective equalization in the inverse, you hear the more pleasing balance you seek, in a repeatable scientific manner. Simple in principle but elusive in practice not only because few of our home systems meet the initial criteria but arriving at the corrective EQ curve isn’t a slam dunk either.
If one is using a preamp with a loudness switch, it is likely to be ineffective because the “inflection point” where the volume and loudness diverge rarely matches the perceived volume in the room with your own speakers. If your amplifier has input gain trims, you might be able to get the loudness in sync with your perceptions, but let’s remember the F-M curves are averages and were arrived at without female test subjects, so YMMV!
Yamaha has a feature on their R-N series called YPAO Volume which uses their room calibration mic to measure the room-speaker response and applies their Variable Loudness with reference to the measured data. This gets you a lot closer to the promised land, imo.
Those of us not using the Yamaha hardware can monkey around with various tone controls…I use a Quad 33 and a Schiit Lokius to custom tailor my low volume EQ.