i had a read through this thread again, but im unsure what it is that you have connected by toslink. could you remind me?
im pretty sure that your computer will only output in 5.1 depending on the program you are using, like xbmc for instance setup to output bitstream audio
i dont have a dm500s, but my vu duo has to be set to output 5.1. and only if its a channel that supports it. i would doubt that you are able to get 5.1 by using the red and white audio leads.
im pretty new to all this and bought a onkyo and tannoy 5.1 setup a few months ago, and its being working flawlessly. and with a little trial and error was pretty straight forward to setup
given that im pretty new to all this, i could be wrong about everything i have said lol
do you have or have access to something you can connect to this that uses hdmi like a ps3/bluray player etc??