The gain or level is the input sensitivity of the amplifier. It determines the amplification of the low level music signal in the amplifier. If the gain is opened too wide, the music signal starts to be overloaded/overdistorted. Acoustically, the audio signal begins to distort. Avoid distorted music signal, this can damage your speakers. By means of the gain control the headunit can be adjusted to the power amplifier, for this proceed as follows: Set the headunit to about 3/4 of the maximum volume, then slowly turn up the gain control until the music signal no longer gets louder and it starts to distort. Once this point is reached, turn the gain control back minimally. Repeat this test with different music titles. After that, the power amplifier should work optimally together with your head unit.