I have a after market stereo in my car with factory sub(just got the car) and im assuming that its hooked to the factory amp. Is there a way to turn down the gain on the amp, it hises at low volume and pops when I turn the car off.
The factory amp is not adjustable. The amp uses the rear door audio as the signal source to feed the sub. Adjusting the left and right rear audio on your head unit is one of the few workarounds. The clunk when you power off the head unit is from the remote power feed telling the amp to turn on and off. From what I gather the OEM head unit had a built in delay for this, where as most aftermarket head units rely on the amp for a soft start and soft power down.
I haven't put an aftermarket radio in my Vibe, but I remember that when people here first started doing it, they said that they needed to hook up a level converter between the new head unit and the amp to get rid of hiss and popping