Remote Amp Volume control

I am trying to have an adjustable volume for my amp that I can just switch between where the deck is set and approximately a volume level of 10% of what the deck is set to. I have seen threads on the line level controllers to adjust the volume of the system. I am assuming that it is just a potentiometer and changing the resistance of the signal that is going to the amp. If this is the case, does anyone know the range of the resistance and/or approximately how much resistance would give a signal of 10% of the high volume setting? I am no stereo guru so feel free to enlighten me. My goal is to have a remote switch to just lower the volume with the flick of a switch when you pick up a rider, etc. Then flick the switch once you are under way and go back to the volume set on the head unit. I know a mute button is similar, but I don't want to completely mute the music, just turn it down. Thanks for the help.

We have 3000W system in our boat, have the regular volume controll but what is sweet is the bass , tower and speakers inside boat are all controlled seperatly