Ya, a gain is there to match the amplifier to various input sources (decks mainly.) Think of a cheaper deck with .5V maximum output and a high-end deck with 8V maximum input. If you didn't have a gain then the manufacturer would have to design the amp for the lower denomenator. The person with the high end deck would max the amp out on a setting of like 5/40. The gain allows you to match the amplifier to the 8V or .5V deck so that at maximum volumn the amplifier is just starting to clip (when properly adjusted.) It is important to note this is the low-level signal and has nothing to do with how loud the amp gets.