rockbox is custom firmware for your ipod that will make it sound much better by taking better advantage of the hardware than the stock ipod firmware.
http://www.rockbox.org/
see if your ipod is supported and follow the instructions there. I used it on my clip+ and the difference was night and day, it went from collecting dust in my drawer to being permanently in use though the difference might be smaller with the ipod as I've never tried this with an ipod. I assure you that with your she3590s and rockbox on your ipod you'll be happy with your sound for a while, it also has a good eq.