If you allow your Blu-ray player to convert DTS to PCM and allow your TV to pass through the audio through HDMi eARC to the Sonos Arc, you should get multichannel PCM 5.1/7.1 from all of your DTS-HD Master Audio and DTS:X encoded discs. And, yes, it will sound just as good as DTS.

As GuitarSuperstar writes, this is correct. I have this set up working with an LG UBK90 Blu-ray/4K player, a LG C9 55” TV (2019) and the Arc 5.1 set up. I have eARC enabled (default is off) and passthrough is the mode for the ARC output. This prevents the LG from taking control of the stream. You want to make sure you blu-ray player is also not mixing audio from the menus. (There is often a setting for this.)

Also, ensure your cable to the Arc supports ethernet over HDMI which is how the audio signals travel. (I cannot recall if I used the cable that came with the Arc - is there one? - or my own.)

There is a thread on this group with the Panasonic 820 4k player where some modes of DTS decoding are not working at the moment, and the suspicion is that the LG TV is the issue. I’ve seen similar posts with other mixes of HW from Samsung and Sony. 

One thing to test if you can is if Atmos is passing all the way to the Arc without being decoded into Dolby Digital 5.1. If the Sonos App shows the Atmos Logo, it is working. That is a good first test on the set up. Then you can debug menu settings for DTS to PCM being proper.

I’ve been diving into this all week, so it’s fresh on my mind! (But working, I am happy to report. My only issue is that with my LG 4k player, I have to turn off decode to get Atmos to work and back on for DTS discs.)