Fletcher Munson 'Loudness Control' Video

My old Acoustic Research amplifier has tone controls modeled off the Fletcher-Munson curves and notably uses sliding inflection points for the equalization -- the more boost was dialed in, the closer the inflection hinge point came to the center.  This was a sensible design that was lauded at the time.

Dr. Floyd Toole covers this subject thoroughly in his book "Sound Enginnering."  One thing he discusses is that experienced listeners are much less likely to boost the bass and treble from "ideal" levels than are novices.  Also, everyone seems to like bass so if any tweaking is to occur even among experienced listeners it is likely to be in the bass region.  In all of reported experiments under controlled conditions novices adjusted both bass and treble to quite high levels.  This from the 4th edition, chapter 12 if memory serves.

Yamaha had it right a long time ago. My 1978 CR2020 had a "Loudness" control knob.

My own familiarity with audio gear dates back to about 1970. At that point and for several decades after that, virtually all receivers (tuner, preamp, amplifier in one box) and all integrated amplifiers had loudness controls. Nothing novel about that. When the purist approach took over the high end, thanks to Harry Pearson, etc, the loudness control became a "quaint" feature that one was taught to do without, because bass, treble, and loudness controls color the sound, we were told.  Soon, they were gone from the scene. And not much missed. 

The RME Adi-2 dac has a loudness button on the remote and I use it a lot