Rrw71- this is a really good idea and your question comes up a lot. From a safety standpoint, I can't stress enough, how important it is to have control of your stereo system. Some people like to have active control of all three zones -- sub, cabin and tower. But at the vary least every tower system should have a way to reduce volume once you drop your rider. Fiddling with your fade on your deck is not a good option.
There are several ways you can add this feature into an existing tower setup, but the easiest is to use a device like a PK-LC1 remote level controller. The part costs 10 bucks on amazon. It remotely mounts on your helm and enables quick and easy control of your tower sound. It would be wired upstream of the amplifier signal path. Very easy to install.
Lastly, it's a good device to have in place so you can blend your tower speakers into the overall sound field while cruising, or ratted up with other boats.
Hope this helps.