Why do I need to turn my amp gain down? It's louder all the way up!!
A#1:
A gain control IS NOT A VOLUME CONTROL. The gain control does not limit the power of the amplifier. A gain turned to 50% does not limit the amplifier to 50% power.
A gain control's only function is to tailor the amplifier's output to the volume knob on your head unit. All headunits put out different voltages, the gain know allows the user to adjust the amplifier output to use the full range of volume control on the headunit.
An amplifier can be pushed to full output or higher(clipping) with input of less than 1 Volt. EQ settings, like bass boost, subwoofer level and loudness controls wil increase the output voltage of the headunit. Every 3dB of EQ gain will double to headunit output voltage.
A#2:
One of the most common install mistakes that leads to audio component failure is setting the gain levels on your amplifier too high. Having the gains set too high results in clipping, which turns into heat, which will in time kill both woofers and amplifiers. The inputs on DD amps accept 6 volts to .2 volts, labeled on the amplifiers gain control/s. If you're using a headunit with a 2V RCA output your gain should be set at roughly 50%, and you're using a headunit with a 4V RCA output your gain should be set at roughly 25%. These settings will vary slightly with every install, but for the majority of installs this is a good rule of thumb. A couple of things that can alter the standard gain settings are the use of bass boosts and the use of different types of source materials: i.e. CD, IPod, thumb drive, radio, MP3, etc as they will all affect your headunits RCA output.