Optical has always worked for me (on a G68AXV), but of course you do need to program that in. I assume you used the config program to set it up or had optical working prior?

Meridian G68 AXV, HD621 | Cinepro 2k6mk5 | 4x Scanspeak A4 monitors, center & 26W Sub | RPi Hifiberry digi+ | Roon