all the zone controllers that I found were crazy expensive and seemed way over complicated.  So i got to designing and built a amp remote wire switch that houses 3 RCA level controllers. it's been amazing to be able to change the volume of the subwoofer, in boat, and tower speakers all separately.  

Parts list:

• 3x RCA Level Controller 
• 5x 12mm Latching Button Switches
• 1x 6 core 20ga wire
• #6 ½" screws
• optional: 6 pin DTM connectors
• 3x RCA Cables to your desired lengths

I wire the red wires of the switches all to the same wire that will be the supply voltage from the source unit to turn on the amps, each switch can represent an amplifier and has it's own wire to go to the amp.  This has been amazing and way cheaper than the options out there from big brands.  It utilizes #6 hardware throughout and meant to screw down to a flat surface.