How did you wire your stereo system?

I'm thinking I need to re-wire the stereo system on our boat. As it is, there are 3 amps: 1 for the original speakers, 1 for the sub, and 1 for the tower speakers. All 3 amps are wired to the stereo outputs. Everything works, BUT... I only have 1 control for the volume. I can't turn the tunes up loud enough for the rider to hear, without blaring the boat speakers and sub.

So, how did you wire your system?

It was suggested that I use a equalizer like this one: Clarion EQS746. With it I can independently control the sub and volume levels, as well as fade to the tower speakers. That seems like the cheapest fix, but is that the best way to go?

Thanks!