In the 1980s, the Bose corporation provided that second control in auto radio systems in select GM cars with the option for Bose loudspeakers and radios. That's what is missing from most two-channel receivers and older preamps with a fixed loudness button. frequency to adjust for the differences in the sensitivities of your loudspeakers and your room size/listening distance. Loudness contours really need two controls: one for 'on' and 'off', and another for adjustment of the degree of boost vs.
