In theory, a loudness circuit is supposed to adjust the frequency response of the unit to compensate for how typical human hearing varies in sensitivity to different sound frequencies by decibel level. In other words, since your hearing is much more sensitive at, say, 2500Hz than it is at 50Hz, the 50Hz signal has to be a lot louder for you to perceive it than the 2500Hz signal. The same is true for treble frequencies, and the function follows the curves plotted by two researchers named Fletcher and Munson (in 1933, if memory serves). These functions vary in two dimensions, frequency and decibel level, so as you turn up the volume control the entire frequency response is supposed to change at each level (or within certain thresholds).
There are three "weightings" that came out of the F/M research, called the A, B and C weightings. A = 40db (like background noise), B = 70db (spoken voices?), and C = 100db (loud). If you look at the curves for A-weighting you need a LOT of equalization to hear 50Hz signals, while at 100db you need none. Treble get boosted less, but still quite significantly at A and B levels.
A lot of recent products I've heard just boost bass, and they seem to center that boost right around 80Hz, which in my opinion sounds really bad. So what a loudness control should be and what it is in a given product are often different. It is certainly not like openings the secondaries on an old carburator, or even moving to the higher lift positions on engines with variable valve timing.
It just equalization for people who think they're not getting their money's worth unless they're hearing their woofers all of the time. IMO.
