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Great question, we'll offer some guidance, and other Community members may be able to weigh in. @Tivoburkee and @makaiguy are Streaming Experts who have a lot of experience here and may be able to help.
The Roku Streambars have fantastic UI performance, they're zippy, and they sound great and work flawlessly with the Roku platform. You wouldn't need the Ultra on the same TV, you could move the Ultra to a different TV or even gift it to someone else.
But, if you want to stick with your Roku Ultra and use a different soundbar, just know that typically ARC will pass through sounds to other connected devices, just double-check with your specific TV model to ensure this capability.
Read up a bit here, under "What is ARC?" And don't forget that every TV enables this in a different way, refer to your owner's manual or the TV's online product support pages: https://support.roku.com/article/360034303013
Let us know if we can be of further assistance and we will do our best to assist you.