PCM audio passthru on Linux

On the windows client, with passthru set up, on my amp, i can see the name of the stream, Dolby or DTS HD, etc.
Under Linux i with passthru setup and Linux Mint 18.1 audio out set to HDMI 7.1, the amp always stays on PCM. i do seem to be getting the correct surround sound. Any way to let the amp do the decoding?