When you can tell every piece of music to choose its own equalizer preset, every single tune sounds at its best for your ultimate listening pleasure. If you use the iTunes equalizer, you'll love this trick; if you have a HUGE music library, you'll love it even more.
Doing it one-by-oneDo a right-click or ctrl+click on the column header of the iTunes table to reveal all the "song attributes" in a drop-down list. Check the 'Equalizer' in the list.
Now you will see a newly added column named Equalizer in the table. In each entry you can see a small button which looks like this. This button allows you to pull down the list of equalizer presets and choose one for that piece of song.
Doing it all at onceWhat if you have a genre album, and you just want to use the same equalizer preset for that particular set of songs? In this case you can do a batch change.
First, multiple-select the songs you want to change. Then right-click or ctrl+click the selected songs and choose 'Get Info' from the floating menu.
A new window containing all the batch-editable attributes of the selected songs will appear. Among other entries you might be interested in editing, the list of equalizer presets that you are looking for is sitting near the bottom-right corner. Choose an equalizer preset form the list, and the action will be propagated to all the selected songs.