Haaris i wud like to add that pre volts mentioned on the headunits mostly are max pre volts out of the head unit, not rms or unclipped voltage 
when pioneer mentions say 6.5 volts preout that is the maximum volts out which the headunit can give. The unclipped or rms could be any where between 0.5 to 6.5 volts, who knows?!
Alpine usually mentions its prevolt output in rms. Even Eclipse headunits are rated to have 8volts preout but in bench tests max they could do was 5v unclipped.
Usually tones / tracks recorded at 0db are taken as reference point when setting gains.
As Sir levin said, distortion can vary from tract to track, depending on what db the track is recorded at. Even if you set the gains perfectly with an O'scope using appropriate 0db test tone, your amp might still clip, maybe because the music track you are playing is recorded at higher dbs. The higher db track wud make the amp to clip early.
