MPRAmpController is a Vapor program that controls the Monoprice "6 Zone Home Audio Multizone Controller and Amplifier Kit". In theory this should also work for the Dayton Audio DAX66 but has not been tested.
Control is done using the RS-232 port with either a Raspberry Pi or Mac and a serial to USB cable. I'm using this one.
This implementation uses ORSSerialPort which is currently unavailable on Linux. As such, this is a Mac only Vapor application.
I did put my previous implementation using the Mac and Linux Compatible SwiftSerial in this repository if you are interested in something that can be potentially run on a Raspberry Pi or similar inexpensive hardware. The implementation using SwiftSerial was working reasonably well on my Mac mini but does have some trouble when used with a Raspberry Pi for some reason. That's the main reason I chose to switch over to using ORSSerialPort.