When you're listening to music through cheap headphones or speakers, an equaliser will help even out the sound. Most equalizers come with genre presets, but you can manually adjust them too. It helps to know what kind of instrument is affected by each frequency.
Blogger Ziyad Nazem lists out the difference between each frequency on iTunes, from the 32 Hz subwoofer sound that many speaker systems can't even play, through the midrange 2KHz frequency that affects the "nasal" sound of vocals, up to the 16KHz that can make a song sound "sizzly." (The list works even if your own software has different reference frequencies.)
To avoid your music clipping (hitting maximum volume and degrading the quality), pull your sliders down rather than up. Then raise the overall volume accordingly.
As Nazem says, there is no "perfect" equaliser setting; the point is to adjust the music to whatever you like, based on the quirks of your particular headphones and chosen tunes. No equaliser setting will entirely "fix" cheap headphones. You'll get the ideal experience by splurging on high-quality headphones or speakers and disabling the equaliser.
The "Perfect" EQ Settings: Unmasking the EQ [Ziyad Nazem]