Sounds like a lot of expenditure with little net gain, and a great opportunity to make the speakers worse. I wouldn't mess with the recipe.
You cannot look at passive prices and brand names and judge their audibility. This is done by measurement, then critical listening, making changes, more measuring/listening... wash, rinse, repeat. That is AFTER talented engineers design a rough outline for the crossover, via sophisticated software. This heavy lifting has already been done
. It is very doubtful that you can improve on their considerable effort with just a few off the cuff passive changes. I have no doubt it can go quite wrong.
I know of at least three gentlemen that do this type of aftermarket crossover hot rod work. You will need to send them one of your Ultra's, so they can do what I described above. That is the only way I could ever recommend a project like this, and it is because they will do it correctly. And, still, your net gain would be a few percent, at best.