Only two possibilities for that:
 Somehow the BluRay player is transcoding the DTS to Dolby or...
 The file you are playing on the USB stick is actually not encoded in DTS... Most Video test clips on the web are encoded as MP4s which is almost always wrapping AAC compressed audio channels in a Dolby configuration in an MP4 multiplex
I can assure you that the HW-K650 cannot process a DTS bitstream - it is clear from the specs in the manual !!!
I have half a dozen DTS blurays and they sound great when bluray player makes surround setup wirelessly. When bluray player creates surround setup, audio settings are greyed out on PCM. If I output audio hdmi bitstream I can select to output between PCM, bitstream reencode dolby digital, bitstream reencode dts or unprocessed bitstream. The dts 5.1 speaker assignment will not work with any of those settings--only creating surround setup with bluray players multiroom link settings plays DTS correctly.
Then in your configuration the BluRay player must be streaming decoded audio directly to the soundbar bypassing the codec limitations of the TV and soundbar - cunning and glad it works !
Out of interest which model of BluRay player are you using?
That depends on whether you like the "faux-surround" - personally (as a film maker) I would always prefer to hear the sound as intended so keep it on the Dolby bitstream setting.
Top of the range player - has codecs for Dolby / DTS and DivX - transcodes from one format to another on the fly (p54 in the manual)
Interesting that this unit also received an over-the-air firmware update on June 7th...
I have also done the TV firmware update and can confirm the surround over WiFi still works fine - phew !!
Just to clear up some confusion from earlier posts:
LPCM vs PCM - In the context of our soundbars these are exactly the same thing. Strictly LPCM stands for Linear Pulse Code Modulation which is the type of PCM used for WAV format audio. (Another type of encoding is Log PCM but that is not used for WAVs)
DTS vs DOLBY 5.1 - The HW-K650 can process Stereo WAV files and Dolby 5.1 Surround files. It cannot process DTS surround, nor can it process Dolby ATMOS 7.1 or other higher channel count formats. But some DVD and BluRay players can be set to recode a DTS output into Dolby for compatibility. The DTS absence in the K650 is simply a cost saving as the DTS technology requires a licence fee payable by the manufacturer and ultimately the end user.
"But I hear surround when I set the TV to PCM" - Yes and No...
The K650 processes true discrete 5.1 channel surround when fed with a Dolby 5.1 bitstream.
When fed with a PCM stereo (2.0) source it synthesises a surround field from the stereo information and presents this on the rears - the same happens if you create a surround group in the multiroom app and play a stereo audio track through the group. The front left and right on the soundbar will exactly reproduce the intended L&R Stereo, the centre channel will be created from the Mono component of the stereo source (mainly dialogue) and the sub channel will pick up the low frequencies from the stereo source. Much of the time this sounds really good but it is not the surround mix as intended by the film producer.
Volume settings - It is likely that when playing Dolby 5.1 sources the volume level will need to be increased as compared with Stereo sources. This reflects the fact that each channel is being fed discretely. But also check that your offset in the TVs wireless speaker manager is not too extreme - if the soundbar is still set low it will exacerbate this. Based on streaming the "Spooks" movie from Netfix last night I was getting excellent surround and clear dialogue with the Soundbar set in the middle position and the rears just two clicks higher.
Thanks for this clear explanation of PCM v DD. This reflects my experience exactly.
Great post Simon, it really summarizes everything about soundbar setup!
scott, your ultrahd player can transcode DTS to dolby on the fly, maybe when you wirelessly connect it to samsung soundbar it knows it and automatically transcodes.
As for lose less formats like DTS HD MA or Dolby THD, the soundbar will play them, because those formats have regular DTS/Dolby tracks hidden inside the stream.
I doubt that the DTS HD-MA audio speaker assignment sound check has a DD audio file hidden inside it.
When I click on information while it is playing it shows it as MVK and DTS.
"As for lose less formats like DTS HD MA or Dolby THD, the soundbar will play them, because those formats have regular DTS/Dolby tracks hidden inside the stream."
I am aware of backwards compatibility built into DD and DTS sound tracks. When I play a DD true hd bluray I am fairly certain that DD plus is what I am hearing. Same thing with DTS HD MA playing as to DTS 5.1.