Connected up my iMac to my Pioneer VSX-2021 (same as the LX55) and played through some 5.1 material but the receiver only playing out stereo? Out of the 3 I picked TV/Sat, plugged in the optical cable into it. In the set up menu I selected Opt-1 as per manual. it's set to Auto Digital surround in the Amp. Yet I am still getting stereo showing in the amp. I can't see the software that says Output 5.1 either in iDVD or VLC...help!