Ok I tried a bunch of things including pulling the entire center situation (speaker, gadgets, clutter, shelf) out and putting the speaker back raised on yoga blocks. I thought that helped but then checked by removing the blocks and wasn't so sure.
I also tried toggling Dynamic EQ and Dynamic Volume, the latter making the biggest difference (thanks for suggestions there). Not much, or any, difference between Light and Heavy settings, but massive change between Off and Light. I can now halve the overall volume and hear dialog much clearer.
I'm a bit surprised placement wasn't more of an issue and may still do the speaker swap just for optimization, but this one setting may be enough to get the subtitles turned off. Hopefully.
Thank you everyone!