I guess it was a dip in the middle. I know I put the highest frequency slider all the way up the top, then the next highest somewhat less, etc. I can't remember if I put the lowest frequency all the way up, but I definitely increased the bass sliders in a curve like the treble. I believe I left the midrange alone, since increasing it usually causes distortion.

Listen through headphones. I have the high-end $40 Sony earbuds with the iPod remote. They have the best dynamic range and richest bass of any headphones I've had.