[popcornmix]

i don't agree, if KODI decode the lossy DTS track you lose metatada information

try 2013 DTS Blu-Ray Demo Disc Vol.17:
- Dredd – DTS Sound Check DTS-HD MA 7.1
- The Hunger Games – DTS Sound Check DTS-HD MA 7.1

try with 5.1 setup speaker wih kodi decoding then with passthrough enabled to the receiver, there is big difference.You lose virtual back surround speakers and DTS / Ampi DSP optimization.

What platforms are you doing this test on?
Pi can't passthrough DTS-HD.
Other platforms can't decode DTS-HD.

So you'd need to test with Pi decoding the DTS-HD and another platform (e.g. Chromebox) passing through the DTS-HD.
You might want to set the Pi to 7.1 channels and let the receiver do the mixing to 5.1.