So I've just obtained one of the new Fire TV Stick 4K Max. I have a Shield TV Pro as reference connected to a Marantz AVR and Panasonic OLED, and this combination plays back all combinations of video and audio that I have, so I know my downstream devices are all fine.
These are the observations / issues I've found so far with FTV.
1. As reported elsewhere, DTS-HD audio does not appear to pass through correctly (to the Marantz AVR), I just get silence, i.e. the DTS core, isn't even ext
The 2023 fire stick 4k max absolutely supports DTS-HD passthrough.
I know it's true because I've seen it for myself. DTS-HD, DTS-X, even DTS-X IMAX Enhanced all bitstream via Kodi v20 for me.
Dolby TrueHD/Atmos, DD+ EAC-3 JOC all come through fine in Emby. I get no audio with any of the lossless DTS tracks, however, so Emby still has some work to do.