To improve the sound of a Denon DCM-370, just unplug the three conductor cable running from the main circuit card to the little circuit card by the headphone jack. Some sort of ESD or overvoltage protection components for the headphone jack cause distortion even at the rear panel output of this CD player. Of course this mod disables the headphone jack, but this is easily reversible.
Of course, it seems possible to change or remove the headphone protection components, but this would be more difficult and perhaps would cause reliablilty problems. I'm not sure what their purpose is.
If you want to get a quick idea of the sonic effect, use the Denon remote and set the output level down to -6. Then turn up the gain in the amplifier to get the same loudness. Removing the cable as described above is even more effective.