This raises the question of why you've installed a sound system that can go so much louder than you want.

Ha! That is kinda funny, isn't it?

Well actually, it's not usually me that has a problem with it. Couple times now on snowboarding trips we've had somebody sleeping in the back of the car, and then the sombody tries to turn up the music a little bit and WAKEUP! It's actually been kinda funny. But, I'd still like to fix it.

My thinking has always been that it's better to have a system that can handle higher volumes than what you would normally listen to. Generally get a cleaner sound that way, and you're less likely to blow anything if nothing is being stressed.