Quick reading, interpretation and summary (I think?)
- Dolby-Atmos requires minimum 2 height channels, DTS-X does not.
- DTS-X can fine-tune voice signals, Atmos cannot.
- DTS-X and Atmos have DTS, DTS-HD and DTS-TrueHD as back-compatible formats because they are not new codecs but extensions to the DTS-TrueHD codec.
The way I interpret the points above is when the Source detects a compatible receiver, all good. If however, a DTS-X file is played to a non-Atmos receiver (or visa-versa), it will downmix to 5.1 or 7.1 DTS-TrueHD.
Thus, going back to Hutsefruts post #13221, there is something wrong I suggest? A DTS file should be downmixed in the Theatre as 5.1 or 7.1 (TrueImage??)