The HD621 will strip the PCM sound and pass it to the G68. It requires your player to convert the DTS HD to PCM.

Vanilla Dts will be decoded by the G68, but the HD versions are not.

Pretty sure you need to tweak your player to do this.

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