Voltage of the output = sqrt(RMS Power X impedance of the speaker) will get you close so you say at 75% volume your head unit is probably only 1/3 of its rated preamp voltage and lets say after you do the math the volts to the speaker should be 45, that still means nothing the box, the space , the signal ( how distorted is it from the HU?) , the state of the speaker new / old / tight etc I mean really... whats the subsonic set to? whats the freq of the box?
but yeah trust your ears and its nothing like tuning a car
