A loudness contour is designed to compensate for the long-recognized Fletcher-Munson curve, which dates back to 1933 and now is memorialized in ISO 226:2003. The problem is not with the equipment, but with the human ear: at lower volumes, bass and treble sound softer, but of course the impact gradually becomes greater as the volume lowers and, conversely, lessens as the volume increases.
If you want to avoid the negative impact of your own hearing deficiencies, turn your volume control to concert levels. If you have a wife and neighbors, however, this may not be always practical. To suggest that your system should not distort the perfect signal at less than concert levels, however, in order to compensate for human hearing deficiencies seems odd. Concert level sound quality can be achieved without corresponding hearing loss.
The observation that the loudness contour needs to vary with the volume seems consistent with ISO 226:2003. I know this is sacrilege, so you dont need to tell me, but the best variable loudness contour I have found is DFX 8 Audio Enhancement. Of course, to install it, you have to use an all digital system, such as Windows Media Player running variable-bit rate Windows Media Files through a high-quality DAC which accepts a USB input. Best $20 I have spent. The other features on DFX 8 are basically worthless, IMO.
