I agree with KenP that direct streaming from the TV to a streamer to your HAs is probably the best approach for better speech understanding. But it looks like the OP had already decided to buy a home theater system instead of opting to buy a streamer, so I’m assuming that the OP doesn’t have a streamer to begin with.
If the content being watched is Dolby or DTS 5.1 compatible, then usually the voices are concentrated on the front center channel for the most part. So that should help isolating the voices for better clarity hopefully. But if still a challenge even with this setup, there’s always an option to increase the volume balance to make the center channel louder than the other channels.
Sometimes, certain movies have really bad voices in situations because they whisper a lot, or simply due to bad mix downs or they have other sounds going on beside speech. I find that turning on subtitle or closed caption is simply the best option in those situations.