What would give the best overall improvement

Nice system and room you have there. Your vinyl playback system as a synergistic entity and no one item is going to be responsible for a quantum leap forward once you get to a certain level (unless you had some gross mismatch of products that was corrected.) Certainly little chance of a product mismatch with what you've got now. I think most products at the thousand dollar price point are all so closely aligned in quality you'll likely be making sideways lateral swaps with little forward progress. When I owned a Scout, I found it VERY sensitive to what it was sitting on and that's the one thing you don't appear to have addressed. I ended up with a Mappleshade type plinth, there are other options as well. The Ginko Cloud 10 comes to mind as a prime candidate. Once you've addressed the platform, save money until you can move to the next level in all things in the vinyl chain, probably starting with a better phono preamp. In my experience, you'll need to move to the three thousand dollar price retail price point to make really audible improvements over thousand dollar products. The law of diminishing returns has a steep curve (sigh.)