I have a few old receivers with this, but it's usually a switch (on/off) instead of a knob in my case. I was taught that this was necessary not because of the human ears' ability but because of the limitations of older speakers.
Old school paper/cardboard cone woofers with the accordian style surrounds without the heavy magnets behind didn't really have the ability to produce low frequencies unless they were given a good push, at which point with regular volume control the mids would be too loud for your initial desire to keep it soft. Furthermore, paper, although this might sound odd, was often heavier than today's poly or graphite woofers, and therefore harder to vibrate at low volume levels.
Paper is also flexible when compared to the new stuff, meaning that the center of the cone moved in and out a lot more than the perimeter when playing at low volumes, making less bass production.
The same was true for old school tweeters, which (instead of silk dome or other new innovation) were basically a smaller papercone driver. Paper is textured instead of smooth which means it absorbs much of the sound it produces, especially at high frequencies and low volumes.
To correct this, Loudness Control was introduced to compensate for these deficiencies in older speakers at low volume levels. Now, with better materials (e.g. injected molded quartz or graphite or poly woofers and silk dome tweeters), the sound reproduction of high end speakers is much more accurate across the dynamic range (meaning whether it's played soft or loud). Therefore the newer receivers do not have the need to add this feature.
Now before I start sounding like I disagree with the original post person, it is true that vacuum tubes in general can produce more accurate sound reproduction, but they have a lot of limitations as far as cost, available power production, maintenance, delacacy, and chance of failure in the middle of a DJ gig. Replacement tubes are an annihilation to one's wallet.
Also, someone who is around a lot of noise often will develop very slight (and not generally noticeable) hearing loss, which starts with deficiencies in hearing extreme low and high frequencies at a low volume. This hearing loss isn't (as originally hypothesized) caused by hearing things that are too loud, but instead caused by prolonged exposure to constant background noise or music (at the workplace or city traffic noise, etc.), whether it be loud or soft.
Anyway, as far as winamp goes, if I want the above mentioned loudness effect, I just go and adjust the equalizer - whether it be the onscreen winamp eq or the eq hardware on the sound system.
Don't forget to live before you die.