Audio Systems: Gain controls, amplifier gain, amplifier output

Question
Hi on the front of most power amplifiers there are two Gain Controls knobs what are they used to determined??? Overall amplifier output loudness or the amount of input allowed or something else???

Answer
Input gain left and right channels which naturally will effect the output gain. If you hear the volume change when you turn them clockwise then they are doing what their supposed to, by controlling output voltage which translates into power (wattage) and forces an electro mechanical device(the speaker) to move air. Technically loudness and volume are not the same. One is a function of airborne sound and how we hear, while the other is a electro-mechanical device to increase amplifier gain.