Hi Bill, Excellent idea. Since the Cornwall's woofer goes up into the midrange using a sub will noticeably decrease distortion. Good subwoofer systems do both HiFi and theater well. The ideal subwoofer system for Cornwalls would require two 12 or preferably 15" sealed subwoofers and a dedicated two way crossover set to 80 to 100 Hz with a slope no less than 28 dB/oct. I would use a digital crossover with room control and delay capability. So, there is no need for fancy electronics in the sub. The best way to do this would be to go to Parts Express and get this, https://www.parts-express.com/Dayton-Audio-15-Reference-Series-HO-Subwoofer-and-Cabinet-P-300-7093?quantity=1
You will need two of them. They are super easy to build and the driver is better than what you get in most commercial subwoofers. The money you save can be spent on the crossover. I would also upgrade the plate amp to this, https://www.parts-express.com/Dayton-Audio-SPA1000-1000W-Subwoofer-Plate-Amplifier-300-809?quantity=1