Yeah, it is definitely a pro/con situation. A part of me would like to be shown rather than told what Kylo has seen that compelled him to take this path, and I think seeing/hearing something would add to the impact of realizing he has been manipulated by Palpatine since the beginning. It would be cool to hear Vader’s voice speak to Kylo at least once before We hear Palpatine mimic it in IX.

Then again, I like Kylo basically praying to his grandfather, and not necessarily getting that response. Leaving it open to the imagination.

I think the issue is that what Vader started that Kylo is trying to finish is left rather ambiguous, and I think it makes it harder for the audience to sympathize with him.

Maybe if you didn’t want manifest a voice or vision, you could give Kylo extra dialogue where he verbalizes his goal more explicitly.

This is what he says:

Forgive me. I feel it again. The pull to the light. Supreme Leader senses it. Show me again, the power of the darkness, and I will let nothing stand in our way. Show me, Grandfather, and I will finish what you started.

See. It’s pretty ambiguous. At the very least you could add one line like this:

Forgive me. I feel it again. The pull to the light. Supreme Leader senses it. Show me again, the power of the darkness, and I will let nothing stand in our way. I will bring order to the galaxy. Show me, Grandfather, and I will finish what you started.

Your Snoke idea is pretty interesting too! I think it would fit in well when he speaking through a hologram, you could have the hologram voice distort and have Palpatine’s voice merged with Snoke’s. Maybe not immediately noticeable but in retrospect it would be another little clue.