Make sure on your Marantz Receiver, within the HDMI Setup, "HDMI Passthrough" is set to on.
As an owner of an Xbox One X and Shield, my experience with both devices suggests that the Shield is far superior, especially on the audio side. Using Kodi or Plex on both devices, the Xbox needs to transcode Lossless audio formats, whereas the Shield passes lossless to the AVR to decode. Using Kodi on the Xbox, my AVR displays Dolby Digital or DTS for my 1:1 ripped UHD or BD movies.
The Xbox, like the Apple TV 4k, uses Dolby MAT, and as a result, Atmos is constantly enabled.
Regarding your Marantz AVR, with sound enhancers such as DTS Neural X or Dolby Digital Upmix enabled, your speakers selected as "Atmos" speakers are constantly in use on the Shield, unless you configure the device to a 2-channel configuration, for example. When watching Netflix on the Shield (the native app), DTS Neural X or Dolby Digital Upmix are used.
On the Xbox, using Dolby MAT, titles that are not in Atmos, will disable your "Atmos" speakers.
Marantz SR7010/Marantz MM7025
Oppo UDP-203 - Nvidia Shield
XBox One X
Klipsch RP-260f - RP-440C - RP140sa- RP-160m - RB10 - Sony SA-WM500
Last edited by groove93; 07-01-2019 at 10:39 AM.