Good info. I however don't have any time for a DYI project as we purchased this old house as a fixer upper. So all my weekend time goes back into those projects. I appreciate feedback. Thanks.
One of the new Gen 2 Mark Audio drivers, is the solution to your problem. We finally a think have a good updated heir to the Jordan Watts full range driver. These Mark units are its direct descendants. I use a single 4" Jordan Watts driver as the center in my Eagan system. It matches seamlessly with the mains, and has very good speech clarity.
This 3.5" driver will give you room to spare, and it would take very little time out from your projects to build a small sealed box for it. You can cross it over at 90 Hz. No crossover or any other drivers required.
This will be far better than any other solution for you problem, especially that Polk speaker.
These drivers should match perfectly any good and well balanced speaker.
An original MK II Jordan Watts modular full range driver.
In my view the most significant moving coil loudspeaker driver since the invention of the moving coil driver.
I think after 60 years people are just waking up to the fact that this driver was and is the correct approach. What has really held up progress in loudspeakers has been the failure to build wide band drivers. Instead we have had a surfeit of narrow band drivers.