Speakers do not get louder with more voltage on the preamp side.

Higher preamp voltages means a stronger rejection of noise and interference on the signal lines which in turn means cleaner sound from the amplifiers. The gain control on the amp is meant to match the line voltage. Which is why if you look at a quality amp, the lowest gain setting will be the highest voltage (ie 7v) and the highest gain setting will be the lowest voltage (ie .5v).

The amp should be matched so that it produces its maximum wattage at almost the highest volume settings on the headunit regardless of the preamp voltage. The problem is that most people are ignorant of that fact and use the gain setting as a volume knob which results in clipped signals and blown speakers.