Like Vermonster, I have sold the Yamaha variable loudness concept for years, and he explains it perfectly. [They actually did make one stereo receiver model with VL on the remote (it had 2 motorized pots), as Jovian notes...this shoulda been the default design]. But I want to mention the approach taken by Crown Int'l with their IC-150 preamp. Like most pushbutton loudness buttons, it grossly overcompensated, making it a glorified bass boost at all levels...BUT...in the manual they explained its proper use...with the VC set to 2PM (where its effect disappeared), the user was to lower the input gain controls on the amplifier (which theirs had) to set the inflection point where the ear heard full range...thus calibrating the loudness button properly, making it much more effective. Also, McIntosh has a variable loudness knob on their C-20 classic tube preamp...and it's the best one I've found, with a well judged contour and that's based on 1960s research on frequency dependant hearing perception.
