I have a Mac Pro and Logitech Z-5500s. Ever since I have had my Mac Pro, I have used the digital optical port to play audio. I was under the impression that this way, I could get 5.1 audio output through DVDs and all content that supported 5.1. However, I recently noticed in Audio MIDI Setup that 5.1 is greyed out and I am not able to select it. Does the Mac Pro support 5.1? If so, how can I get I get 5.1 output from my MP? Thanks in advance for the help!