Asus Xonar DG: No Sound from Rear/Central Speakers

Hello Tom's Hardware,

This is a problem that I can't seem to find a solution for on Google, which is a very upsetting first.

I recently bought the Asus Xonar DG 5.1 card (http://www.dabs.com/products/asus-xonar-dg-5-1-pci-gamer-soundcard-with-built-in-headphone-amp-to-power-up-every-detail-in-game-75TF.html) and set it up into a new PC.

I tried plugging an optical cable between the sound card and a home theatre system (Sony S-Master STR-KS1000: http://pdf.crse.com/manuals/2661769142.pdf).
However, when trying to play sound from the system, only sound that is supposed to come through the left or right speakers can be heard. Any sound that is directed to the central, rear left, rear right or subwoofer speakers cannot be heard.

The drivers are installed and up to date. The utility provided hasn't got any glaring errors in the settings, that I can see - the "SPDIF out" option is checked, for example, which is a common error my googling has shown. The default sound device in Windows has been set to "Speakers", which is another common error I see around, although people with this problem report that sound that should come out of their rear speakers instead comes out of the front speakers, rather than nothing happening at all.

Here's a screenshot summarising my previous paragraph: http://oi45.tinypic.com/2dhzku8.jpg

The speakers do work as I can use the very same cable and have all the speakers working on other devices that I own.

One thing that comes to mind is that perhaps the home theatre system somehow doesn't support the input coming from the computer, but I doubt that. Given that sound that should be coming out of the rear speakers, etc, is silent, it leads me to believe that this sound is being sent to the right places but is getting lost on the way, somehow.

Any help would be appreciated!