Nowave is essentially correct. The "Loudness" control, offered in the past on Yamaha preamps and receivers and some H/K units was an extra control, which, if it was reasonably well designed (the "contour" of the control), was intended to compensate for our ear's decreasing sensitivity to low bass at quiet listening levels. You first set the volume to a normal fairly loud level. Then you'd turn down the separate Loudness control to whatever low level you preferred. As you rotated the control, it applied gradually increasing levels of bass boost which approximated the loss of bass sensitivity of our ears at low volume.
Most consumers never really understood the purpose of the control, so they gradually disappeared from consumer gear. And some applied increasing bass and treble boost (the treble boost wasn't needed). Other receivers had a cruder "Loudness" button, which when activated, applied a fixed level of bass boost--typically at least 10 dB in the low frequencies and 3 to 5 dB in the highs.
Consumers who bought small, crummy bookshelf systems in the '70s and '80s would invariably run their systems with the Loudness button always activated because it boosted the fairly non-existent bass and poor treble of cheap speakers so they sounded "better" to their ears. With good speakers, the effect was grotesque. When partying, turning up the volume with the Loudness button activated would invariably drive the low-powered receivers into clippiing and burn out woofers or tweeters. For example, making bass subjectively "twice as loud" (an increase of 10 dB) requires ten times as much power, so it was no surprise when inexpensive low-powered receivers blew out speakers driven with the Loudness button activated.
Our ears' insensitivity to deep bass at low volume have been measured and graphed by acousticians and researchers (the first were Fletcher and Munson; later research refined those curves). I suspect you could track them down with a Google search.
Regards,
