You can NOT control the amp or the matrix other than using the Crestron processor you already have.

There was a thread -quite a while back- where someone started implementing a Pronto as Crestron touchpanel. Control is via TCP/IP directly to the processor. You don't need to use the way around serial.
This might help you if you don't consider buying Crestron touchpanels.

Another option is to use an iPod Touch (or iPhone) with the third party App "CommandFusion" that controls your Crestron stuff wirelessly. iPod Touches are darn cheap for that! Crestron is supposed to have native support for those Apple devices in Q1-2009. More info to follow after ISEurope, February 2009.