Have you ever think about modify your crossover?

It's an option if you don't like the x-over point or slope used. You could build a new one to cross a little lower or higher and run a steeper slope or similar. I doubt you'll find much of a difference just changing to higher quality components alone. The real difference will be in what x-over point and slopes you use. As well, if you're this interested in modifying your x-over, you might as well consider just going active and get full control over it. There are some pretty affordable high end HUs available that are up to the task. It's just a matter of if you want to swap out your stock HU...if you even can.