I recently purchased a set of Logitech Z-5500 speakers. I have the proper optical cable, and all my speakers are set up properly. For whatever reason, I can't get 5.1 to output from my Macbook Pro when using iTunes. Audio MIDI Setup is currently set to 44100.0 Hz 2ch-32bit digital output. I can't change it to 5 channel. (I don't know anyone who has to, anyway.) Any ideas?