http://en.wikipedia.org/wiki/Overdrive_%28music%29

"A guitar amplifier's tone controls shape a different power-tube distortion voicing if the tone controls are set to emphasize the bass or treble. Extreme settings are most popular in heavy metal. Increasing the bass and treble while reducing or eliminating the centre midrange (750Hz) results in what is popularly known as a "scooped" sound (since the midrange frequencies "scooped" out). James Hetfield of Metallica used this tone on many songs on Metallica's first four studio albums. Conversely, decreasing the bass while increasing the midrange and treble creates a harsher sound. Kerry King and Jeff Hanneman of Slayer have both used midrange-heavy tones since the mid-1980s."