This has been a journey, more than just a project. The question is asked frequently, "Which Center Speaker matches the Vintage Polk Models". Polk has recommendations, and I believe some of them now qualify as vintage too. This all started when I bought my Monitor 10s. The center I was using in no way matched the timber of the 10s. I've since used CRS+ and now use my SDA 2A-TLs as my fronts, for 2 channel and Home Theater use. I initially built a proto type center, for experimentation, never intending for it to be permanent. I used many different combinations of Polk drivers, Crossovers etc. It's been vented, PR'd, and actually sounded quite acceptable, even though it was undersized and aesthetically challenged.
Since I've now completed all the upgrades to my 2As, and have re-configured my 5.1 to a 7.1 system, using my Modified Monitor 5s and 4.6s, it was time to get serious and build a proper Center, that will match the 2A-TLs, and look professional.