I just received a personal present to myself, the Logitech Z-5500 5.1 speaker set. This set, for those that aren't familiar, does DTS decoding and accepts optical and coax input via a small control center that sits on your desk. After hooking everything up via my PowerMac's optical output, everything sounded great but I couldn't get anything but a stereo output. After some searching I found out that even the PM optical output is (apparently) downmixed to two-channel unless you change some settings. DTS output doesn't work at all. Step 1: From System Preferences, select "sound" and Digital Out as your method of output. (Next two steps, close all sound-related programs such as iTunes, DVD Player, etc.) Step 2: Set your system-wide output settings to do digital encoding. This is from Utilities / Audio MIDI Setup. You want to change your output to Digital Out and the Format from downsampled 2-channel, 16-bit to "Encoded Digital Audio." Step 3: If you want to view DVDs with DTS, load up DVD Player Preferences. Change Audio Output from System Sound Output to Digital Out. After all this your system will now send out the proper digital signals for your receiver to decode. The final output on my PowerMac went from "wow" to "OMFG!" This web site shows the final two steps with pics. I have to say that this is the first counter-intuitive thing I've had to setup on my Mac since I got it. These settings are well hidden and enabling the optical port is not enough to get DTS output. Hope someone finds this helpful.