Hello Andi,
Thanks for your response. I have spoken to Sibelius tech line this morning ... interesting.
It turns out that (apparently) one should not use the soundset for Plus if one has not loaded any Plus instruments. My score has the PLus soundset but no Plus instruments (saving on RAM).
Now, I was told (by you or possibly Paul) that the Plus soundset includes the Special Edition soundset and therefore (I thought) it was okay to just work with the Plus soundset - especially if I later decided to change solo violin to solo violin plus in a string orchestra + solo violin piece because I decided I wanted the extra variety of articulations (all this on one instance of Ensemble). Unfortunately it seems that using the Plus soundset when no Plus instruments are loaded can cause confusion in Sibelius. In which case it seems one must open two instances of Ensemble, one for PLus instruments with the PLus soundset, and one with Special Edition instruments and soundset. It seems you can't mix them on a single instance of Ensemble. Or at least this is as much sense as I can make.
I have three soundsets for Vienna: one for Special Edition Plus, one for Ensemble and one marked VSL. If I have no Plus instruments it seems I should use the Ensemble soundset (not sure what the difference is between it and the other two).
It seems the orchestral double bass is not available on the list of instruments in the Manual Soundset list in Sibelius PLayback Device for when one is creating a list of channels / instruments, when using the VSL soundset.
Whether changing the soundset will solve my playback problem I will let you know when I have a chance to check it out later today.
I hope these posts will be of use in clarifying these issues for others just starting out.
Best wishes,
Rikky