[mrchrister]

ok, some news in the meantime: i tried out mrmc and lo and behold, dts passthrough works as expected! I remember reading somewhere that the dev had an old denon receiver. for some reason whatever he changed made dts work for my firetv 2
I'm now back using SPMC though because it works better. mrmc had issues with stalker and av sync issues.
any chance to get dts fixed at some point maybe?

@Koying just found the explanation why it's working:
That is because your pc/laptop does passthrough in a very different way than android. DD works because fireOS supports ENCODING_AC3 and ENCODING_E_AC3 in AudioTrack. ENCODING_DTS is not a supported format so we try the age old trick of faking it as PCM. This work provided the avr is not super picky like those denons. Works with my old denon x1912. Exact same setup fails under my denon x3200w. It's been a huge problem among AppleTV4 users that have those receivers.
https://forum.mrmc.tv/viewtopic.php?f=17&t=725&start=40