[himey]

Yeah. When I developed the v19 audio improvement I had the firetv with the firmware out of that time. They fully supported the standard Api. So all this code was merged into kodi.

So, if you want that kodi does the job, use v19 or fritschfiretv build, NOT the official v18.
You have to choose the RAW PT device in kodi settings for audio.

Set speakers to 2.0
Enable dolby transcoding
Enable AC3
Enable dd+
Disable all the rest.

In firetv audio settings choose best available.

That way: dd+ will directly be output, AC3 as well, every other format including lpcm will be transcoded to AC3 by kodi itself.

Obviously you wil loose real lpcm output.

For the firetv cube I found lpcm still working nicely. On this device I plain disable passthrough and let the amazon FW do their dd+ encoding...

If bit perfect is needed get another device 😉

Giving up multichannel PCM formats like 5.1 lpcm, dolby true HD, and DTSMA so you can get ac3 and dd+ working is a huge tradeoff. I will stick to v18 when using my Firestick 4K. Also, PKC doesn't work using Matrix.