Hi David
Here is a fedback from my side which trys to give answers to your titled question. Keep in mind that notation prorgams do what they are programmed for. Take for example the score of the Violin Concert BWV_1041 and let the program play those notations. It never will sound as you are used too. The very first reason is, that the program not will take into account that it should play in "Baroque Style" which is a matter of "short & long", "heavy & light" and more.
OK, you could set those things with signs within the notation but nobody is used then to read such notations. For musical results you also should know that one should choose the right amples not bei their names but by their sounds. So I often use a long "diminuendo" instead of a sustain because the diminuendo starts with a nice swell in its first part which is often better than a flat sustain sound. In a Daw you can do all those things better and often with a better resolution so that it can lead to such results: BWV_1041_Violin_Concert.mp3
When I say "in a better resolution". Whatch please this short video at youtube about staccatos. How would you program all these small different settings for the volume parameter with a score program?
So be happy to have a meaning how your music could sound with your sibelius.
But if you want musically results as well you probably need to do this with a DAW and ideally of course with full libraries for having a lot of possible articulations handy.
When you ask "Why do the solo strings demos sound much better than what I get in Sibelius?"... what means better? So it also could be that the final result sounds different because of a better mastering...
Best
Beat
- Tips & Tricks while using Samples of VSL.. see at: https://www.beat-kaufmann.com/vitutorials/ - Tutorial "Mixing an Orchestra": https://www.beat-kaufmann.com/mixing-an-orchestra/