I have a MacIntel Mac Pro 4 core machine. It has a built-in speaker out as well as SPD/IF optical I/O. What would I need to be able to monitor source in 5.1? I presume the SPD/IF will output a digital 5.1 signal. Do I need to then get a 5.1 amp to take the optical output and feed it to a surround speaker set? If so, what one would be compatible with the SPD/IF ports?

TIA!

Take a look at the Apogee Ensemble http://www.apogeedigital.com/products/ensemble.php

Once you get one of those you can jump into your Utilities>Audio MIDI Setup and change your default output to the Ensemble. And "configure speakers" to multichannel

Well, I was hoping to levarage the SPD/IF, not the firewire+$2K in hardware :)