You must use the input that you have assigned the remote control to. I am not sure if the Mk3 has the same menu as the Mk1. Maybe you can get hold of a reference manual?
Below a couple of screenshots of my settings:
I assigned the S/PDIF input to the TV command on the remote:
Then under OPTIONS/SOUND, you can see how I listed my speakers, their roles, and their volume & distance settings, also that I want to use all speakers for video use, and 2 speakers for audio use (SPEAKER 5 / SPEAKER 2)
Are you not using a centre speaker? Could it be to better use another powerlink socket? The first two are dedicated to center and sub:
