Thanks very much for all the help. I think we pretty much have the answer, thanks to the help! 
@TonyPh12345 was confused and rightly so. In my first post, I said that both our Apple TV and WD TV were passing through our trusty old Pioneer receiver.
I corrected that in my second post…
I just realized that we run our Apple TV hardware through the receiver, but the WDTV goes to our Samsung UN65KS8000. We do that because the old receiver won’t play the WDTV (a different issue - HDCP).
So the WDTV HDMI-out goes straight to the TV, Tony (i.e., to the Samsung), and then from there to the receiver.
EXTRA INFO (might be relevant, I don’t know): The connection from Samsung to the receiver runs via Optical Audio because the Samsung One-Connect does not seem to have an HDMI-out.
@TonyPh12345, I hope that clarifies. Sorry for the confusion.
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@TonyPh12345 also suggested setting the WD’s audio mode to STEREO. Clever idea! Yup, when the WD sends stereo to the Samsung, it plays sounds just fine. This confirms the guide’s documentation that Samsung won’t play DTS-MA.
I could have sworn that we did play our first few movies, and reviewed again with my wife which vids we had watched, that we had played DTSMA vids with no problem. But there are so many options, we could have played “London Has Fallen,” for example, through the Apple TV option in the Samsung menu rather than from my copy of the file in the hard disk attached to the WD TV.
I was hoping that @ERmorel was right when he said…
This version of the media player should play DTS files in 2 channels without the need for an audio receiver that can decode the codec. Since you have a receiver, you shouldn’t have any issues.
While it wasn’t going to “a receiver,” I would assume that if it did play DTSMA in 2 channels (I don’t know what that means), then the Samsung would be playing those vids and would not have been displaying “DTSMA” in a little black box in the top left corner of the screen. However, it was seeing (and displaying) DTSMA and it couldn’t decode it - the fact that it played the same vids when the WD ouputs it as STEREO pretty much confirms that, it seems to me.
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Bottom line: Looks like I need a new receiver. That way, I won’t have the HDCP issue with WD’s connection to the old receiver and I can run everything through the receiver. The lack of an HDMI-out was sub-optimal anyway (i.e., having to use an optical cable for sound-out).
Thanks very much for the help, especially to Tony. 
Please let me know if I am drawing any wrong conclusions or if this is truly the bottom line? I’m at the very limits of my understanding.
All the best,
Ken
I guess a second bottom line is…
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Samsung should add that codec - it’s common.
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HDMI-out would be nice. It’s kind of strange not to have an HDMI-out because in our second home, I run everything through the TV first and then via HDMI-out to the receiver (HDCP issues). The absence of an HDMI Out reduces setup flexibility, it seems to me.
Just suggestions, made in good spirits. I love the TV. 