What are you using for an audio interface? That is the best thing to route your audio out to your speakers with. I've not had much luck with pcie type soundcards and pro tools. Last I remember it's not advised. Tentative, at best.
Im sure you use some sort of audio interface to get your signals into pro tools.
I say your best chance for success is picking up an audio interface if you don't use one.
Hope this helps. Also, if I'm wrong about any of this, I look forward to being corrected. I run pro tools 12.3, and not too familiar with newer pt 2018 logistics, if there are any.
Cheers