Dolby Atmos Passthrough not detected by amp

[Napper1327]

Hi all

I'm using an H8 OctaCore - latest firmware, KODI 17 Krypton (or SPMC or PLEX).

It's getting media from my QNAP TS451 over CAT6 and going into a Yamaha RX-V583 HDMI.

I've set the audio out on the H8 to HDMI, KODI (and SPMC) system settings > Audio > Enable Passthrough

The amp is set to receive the signal as sent ('Straight' ie no DSP/Virtual surround). I've checked with the manufacturer and that's correct for receiving an Atmos signal using passthrough. the amp is configured correctly to output in the correct 5.1.2 setup

When I play any of the Dolby Digital theater test files (5.1 DD, TrueHD7.1, Lossless Atmos etc) the amp always detects the signal as PCM 2/0/---

What am I missing? Any help would be soooo appreciated, I'm going a bit mad trying to figure this out.

Thanks

N