
Brian W. answered 10/20/20
Career Songwriter
Hi! Wish I saw a name attached to this question. NOTE I am answering from a Songwriter tag but my goodness this is mostly a Production/DAW tag xD
Answer: You don't like the sound of the virtual instrument (VI). Sibelius is notation software using low-quality virtual instruments to playback your creation. Reaper is a genuine DAW, but even the built-in Reaper VI isn't that great (no offense intended). In fact, most built-in DAW VI is decent, at best. Logic probably comes closest and has some powerful built-in engines that makes it easier to start from.
IF you want great sounding VI, it's something to invest into. I'm unaware of how to use VI in Sibelius, but I imagine it's not possible. You could try notating music in a DAW, I know Logic has some tools for it that I've used. But the long and short of it is as long as you are limited by the VI, you won't enjoy the way it sounds.