Turn it up until the sound distorts, then back it off one.
Or, just go to 11.
Mine: 2002 Focus SVT: 137k, Sonic Blue, #1654, Moonroof, Winter Pack, Audiophile
Mods: Tom's Tune, Borla, CFM STS, painted Machs, Euro grille
Hers: 2010 Focus SES: 59k, Ebony, Auto, Moon and Tune