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. The current implementation has some trouble when used with a Raspberry Pi for some reason. I hope to have this worked out because I'd prefer not to sacrifice my Mac mini to just control this system.