If I turn my gain all the way down on my amp how much decrease can I expect in power output? My amps RMS rating is right at the recommended input power for my sub. I prefer not to overdrive it so I would like to use the amp gain to limit the power going to the sub.
You set your amps gain according to the voltage outputs on your HU.
If your amp's gain has V markings, you're set other wise, do this.
Turn your HU up until you hear your speakers start to distort. At that point, turn down a notch at a time, until the HU is playing as loud as possible without distortion (audible).
Next go back to your amp, and increase the gain until you hear the subs distorting. Turn the gain down until the distortion is gone. That is the way to "listen" for the gain settings.
Although you could do it with expensive equipment...But that's the simplest way. Check your gain thought and see if it has Voltage markings, as you'll want to move the gain equal to the Voltage on your pre-outs on your HU.
Yea, like Tyler said, the first thing I'd do was look at the Voltage markings on your knob and set it accordingly to where your HU pre-out voltage would be on the wheel, then turn it like a smidgen down. A shop would probably set your gain exactly for you with an oscilloscope. Also, as long as your power is clean and not clipped, you won't mess up your sub. (Also assuming you're not over excursing your sub in a ported box.)
Isn't clipping sending DC (right?) current from the HU, to the amp, and then basically straight to the voice coil, which is no better than running your sub directly to a wall socket (only in a slower, but steady deterioration)?