Plex won't work for DTS passthrough since the hardware of CX itself can't pass the DTS signal through. What Plex will do is convert the 5.1 channel DTS to another compatible format using your Plex server. For example, most TVs can't decode TRUE HD master so 5.1/7.1 channel TRUE HD will get converted to 5.1/7.1 channel DD+/AAC depending on the TV's compatibility.
Anyway, this will only matter with very high-end audio equipment. Most of the <1 lakh equipment won't be able to resolve even the regular DD to its full potential, let alone DTS or DD+.