That is a whole other topic. That is not terribly abnormal, but maybe you have slight ground issue. When I say issue I mean it is not a good ground or maybe not enough. If you never see your voltage at 14v or higher when at idle and no music playing then start there first. You should see the same voltage reading at the amp as you do on the battery when the car is turned on and no music playing. If not the ground is most likely the reason. Take a volt meter and check the battery terminals when the car is on. If it is 14.xx v then check the amp inputs and see. Music is dynamic so blasting the woofer will make the voltage go where ever the amp needs to take the woofer.