You've probably seen the button
marked 'loudness' on car stereos and older hi-fi equipment. You have probably pushed it as well and noticed that the sound seemed louder, or at least more bassy. No surprises so far. But why bother putting a redundant feature on the stereo - the same effect can be accomplished by turning the volume control
Almost right. We have to go back to the thirties and the expansion of the telephone network to properly evaluate the function of the mysterious button: Bell Labs wanted to send several telephone conversations on carrier waves of different frequencies and needed to know how much bandwidth a conversation required to remain intelligible. Tests were made by Dr. Fletcher and Dr. Munson where people were asked to judge when two sine-wave tones of different frequencies were perceived to be of equal loudness, thus mapping the sensitivity of the human perception of sound. The result indicated that the human hearing was non-linear not only in frequency (test tones in the range 1-5kHz were perceived as louder than tones of equal level outside said range) but also in amplitude - the differences in perceived loudness diminished as the overall loudness of the test tones increased. A diagram illustrating the relationship between percieved loudness, actual loudness and frequency is called a Fletcher-Munson diagram, after the doctors who mapped the deviations of percieved loudness.
An effect of this is that the percieved frequency distribution of a recording played at low volume is different than the perceived frequency distribution of the same recording played at a louder volume - the peak in the range 1-5kHz gets flatter the louder the recording is played.
Most good classical recordings are meant to be played at a realistic (concert hall, first row) volume, where the peaks reach about 110dB. In densely populated areas this is not always advisable and your neighbor might not enjoy percieving your recording regardless of frequency response and tell you to go ride your valkyries elsewhere. But as you lower the volume you notice that the violins, having their sonic habitat at about 1-5kHz, seem to 'stand out' and the basses and kettle drums seem dampened. Enter the loudness button. When the loudness function is activated the frequencies below 1kHz and above 5kHz are boosted (+6dB seems to be a fairly common loudness gain), the dampening effect caused by the nonlinearity of the ear is countered, the perceived frequency distribution is equal to that of the same recording played 30-60dB louder but the actual increase is somewhere between 3-6dB. Drums! Contrabasses! Joy!
Sources: Music and the Human Ear, http://www.silcom.com/~aludwig/EARS.htm
The Numbers (and Initials) of Acoustics, http://arts.ucsc.edu/EMS/Music/tech_background/TE-02/AcNumbers/AcNumbers.html