rocknrollfrenchfries,
If you listen to much current music your problem is probably related to the
loudness wars. Without any remaining headroom in the audio file any
positive gain with the EQ will lead to distortion.
There are two easy ways around this:
1 - Reduce unwanted frequencies instead of boosting wanted ones, and up the volume. (Instead of the stereotypical bass and treble boosting "smiley" EQ, reduce the midrange with a wide filter (small Q value))
2 - Use the Pre-cut Rockbox gave you! If you're adding 15dB of gain with the EQ you really need to set the Pre-cut to -15 dB.