Can't help much I'm afraid, but I know my panasonic plasma can send different signal stuff to one of it's scarts. It's accessed through the TV menu.
Surprised you only have optical input for audio. Do you have a scart from the HD box to the amp? That should be carrying audio with the surround stuff encoded.
This stuff can be a bit of a black art. Once it's set up I leave alone and then forget what's what when I'm ordered to change it around!
Have you looked at the AV forums site? Lots of info & knowledge there, especially if you know the model numbers...
Cheers!