Strange DTS behaviour

I originally encountered this problem with DVD Shrink but after failing to solve it have since spent hours with other products - and found the same thing. I am now even more puzzled, especially after finding similar posts on the web with contradictory information.

DVD player connected to DTS certified amplifier via Toslink.
Play a DTS encoded DVD.
Observe correct DTS decoding as confirmed by the amp automatically switching to DTS.

NAS connected to Boxee Box media streamer connected to DTS certified amplifier via Toslink.

Using any software (DVD Shrink is one example) make a copy of the DVD to any format, being sure to only include DTS 5.1 and not PCM stereo.
Play the copy on a laptop that doesn't support DTS.
Observe no sound. Perfect - as expected.
Stream the copy from the NAS to the DTS amplifier via Boxee Box and Toslink.
Observe sound (which can only be DTS) - but the amp resolutely refuses to display "DTS" and the sound quality is thin with back-to-back comparison against the DVD.

I do not therefore believe that the copy is DTS.

Swapping the Toslink cables for Boxee Box and DVD player confirms that I do not have a hardware issue.

What is going on please?

Many thanks!