I got my Arc yesterday. Connected via HDMI ARC and no passthrough at all from LG C6 TV of anything other than DD5.1.
Not entirely unexpected but really disappointing nonetheless.
I have to say though even 5.1 sounds fantastic compared to my old Playbase.
I tried the secondary HDMI Audio only out from my Oppo player and get no audio from this (nixing at least one cunning plan). As others have said the Arc needs the handshaking that HDMI ARC does and not just a HDMI out.
I have lots of HiRes sources capable of putting out Dolby Atmos but the Arc will only take a signal via my TV which is the bottleneck. I am not replacing the TV anytime soon so I have to hope the LPCM update might give me access to some of what the Arc can really do.
Till then - it’s giving me some of the best 5.1 I’ve heard in a long time...