Audio Systems: loudness control vs bass control?, fletcher munson, infinity primus

Question
I have a Sony stereo receiver from about 1992 connected to Infinity Primus P162 speakers.  I was wondering what the difference is between the bass control knob and the "loudness" control knob.  I usually have them both turned up when I listen to music to squeeze more realistic bass response out of my small speakers.

Answer
The human ear has been analyzed in the real world of acoustics. Instruments used to measure the sensitivity of typical human hearing have determined at what frequency and amplitude the receptors in the ear respond to wave propagation. Usually below 75db the ear is considered to be less efficient in the reproduction of hi frequencies and low frequencies. Two scientists at Bell labs in the 30's were Fletcher and Munson and are renown for naming the Fletcher Munson curve to describe the inefficiency of the human ear as well as the compensation needed to correct this natural phenomenon. It is a compensation curve which increases the bass and treble proportionately as the acoustic amplitude or volume is reduced. That is what the loudness control or contour does. Bass and treble controls, on the other hand, are what we call shelf equalizer curves that are fixed at specific frequencies