I understand clearly what you are trying to do.
First no unfortunately the Sound Enhancer is either on or off and cannot be turned on for each song.
I have played with the Sound Enhancer extensively and honestly on some songs yes it may "seem" better, but honestly overall its not. Voices do not have the clarity, richness in tone, and simply naturalness when its engaged. So personally I would not recommend using it.
I would also recommend rather than using the equalizer presets to manually adjust the equalizer settings for each song which can be done. You simply adjust the equalizer settings and then make a preset (good idea to name it the same as the song so you can reference the equalizer setting to the song. Then simply select that equalizer setting for the same song.
It is however very important to know how to use a equalizer so you get the best sound, and do not risk damaging your speakers, amplifier or both.
Equalizers should be used to "reduce" problem areas in the frequency response that is coloring the sound in a negative way, NOT to boost (unless its very little) . If you boost to much you put the speakers and or amplifier at risk of damage.
I hope you burned your records at the highest bit rate possible that also makes a huge difference in sound quality. 48 kHz 24 bit wave is the highest sample rate you can use in iTunes. You can then create a duplicate sound file at a lower bit rate for your ipod if your limited on hard drive space.
But understand Mp3 is very limited in sound quality and yes it takes up very little hard drive space but it comes at a cost in sound quality.
Kevin
40 years high end audio video specialist