Hello Rob!
Thanks for the screenshots.
I strongly recommend using Vienna Ensemble when working with Sibelius. It will make everything easier, because you only need to activate one playback device and can open the Vienna Instrument players inside. You can install Vienna Ensemble with the Vienna Assistant.
Furthermore I can see that you didn't load the correct VI presets. Our Sibelius sound sets can only work correctly when you load the according VI presets under PRESET => Sibelius. The VI presets for Solo Strings I (full) can be found under Sibelius/Strings 3.8/Solo Strings/SS I - extended
It shouldn't be necessary to set up anything on the "Preferred Sounds" page in your case.
Best regards,
Andi