I not up to speed on the Roku but are you sure your not just up converting the stereo feed from the app internal to the Roku (The Sheild has this option to up convert stereo to dd/dd+). My Sheild just gets PCM stereo for the CBSAA app with the upconvert option turned off of course.If you are paying for the commercial-free version, in the CBSAA Roku app, you should be able to hit the star key and toggle audio options: DD, DD+. The Roku is my go-to for CBSAA as it's the only way I can get DD+ (or any 5.1 audio) on almost all their content.
My subwoofer definitely got a workout during the episode, but I didn't find it overwhelming or problematic. Lot's of activity in my rear speakers. A couple times I looked up to see if someone was outside my house.