The limitation may be the imput on your receiver/amplifier. I know that one of my optical feeds on my tuner, dedicated to video, only reproduces stereo, while the optical feed for audio produces 7.1 sound.
While there are some works produced in true 7.1, the majority of music is produced in stereo and is translated into 7.1 by circuitry.
I don't have an answer for you. I'd look at the specs (if you can get them) on the audio production from the GTX 560 ti and compare them to the specs of the Onkio.