You cannot connect speakers to an optical audio output. The Crestron boxes provide surround sound processing, input selection, and power amps to drive the speakers.
You could replace the Crestron boxes with a surround sound receiver, There are good sounding receivers from Marantz, Anthem, & Cambridge Audio that would update the system without loss of sound quality.
If you have a Crestron home automation system they would have to integrate the receiver and reprogram the system. If the Crestron is just in that room I would just get a new universal remote.
Thank you for your response! So if I just buy something like this:
I can plug in my dvr, ps3, and chromecast into that via hdmi, and then connect the speakers and power amps to the portion labelled "Speaker Systems". Is that correct?
Side question: why is there a + and - in that section? I have a 5.1 system, so where do I plug in my six audio cables?