The Linn DS Player can playback (44.1 kHz to 192 but not DSD or 352)
If I select Mode: Specified Output Format Only When necessary: If plays everything but converts everything.
The problem is with the
"Only When Necessary" part of your statement...
The UPnP renderer implements a method called GetProtocolInfo which returns an offer list comprising all the music formats (mime types) that it is capable of receiving (i.e. playing). And the UPnP control point should parse that list and choose from the offered formats one that the server can deliver. And in cases where there are several possible formats that both the server can deliver, and the renderer can receive, then the control point should choose the one having the highest resolution.
Now for reasons of compatibility (by tradition if not by specification) the renderer's ProtocolInfo offer list usually mentions the "standard" formats L16 (aka 44100Hz 16bit 2ch) and MP3 first, and all the more exotic formats after.
So probably, either a) the Linn is not mentioning in its offer list that it can play PCM up to 192000Hz, or b) it
is doing so, but MC cannot understand the mime descriptor, or c) MC sees that L16 is offered at the start of the Linn's offer list, judges that this is good enough, and does not look for higher resolution offers later in the list.