Sound source output voltage The sound source output voltage is a fixed level unless your sound source with a "variable output". The 2-volts signal (music) drive the power amplifier input stage or drive the preamplifier. It in turn drives the amplifier preamplifier input or power amplifier input stage.
Pre AMP Amplifier
input sensitivity Simply speaking, amplifier input sensitivity means how many volts of level signal are transmitted to the power amplifier. Any voltage value above this amount will cause your power amplifier to try to use more power than it actually has, which will cause it overloads and result in so-called "clipping".
So, a preamplifier is of course used to control the voltage from sound source in all cases. When the preamplifier volume knob is turned to left at its maximum, you can measure that there is only zero-volt voltage output, so there is no sound from power amplifier. When you turn the volume knob to the right and increase the output voltage, your power amplifier can drive the speaker to make a sound. The ideal working range on the volume control should be between 1/4 and 3/4 to the right, which is normal position of the listening level (actually beyond 1/2 to the right, the distortion already exists). That is to say, the preamplifier will never add any voltage to the input signal, which means "gain".