In your case adding a subwoofer is a bad idea. To improve the sound quality you would need to add two 15” subs and a two way crossover. This is much more than you wish to spend. One sub will not keep up with your Cornwalls.
Add a subwoofer?
I run a pair of Cornwall 1’s driven by primaluna, VPI, and Sutherland 20/20. Room is 34 x14 x 14 high. I love the Klipsch, but never felt like they were overly heavy bass. I’m curious about a subs ability to “fill in” the sound. I called a dealer regarding REL’s smaller products. I was told that the Cornwall has huge bass and I’d need a very large sub to notice a difference. Really can’t afford a quality big subwoofer. ie thousands of dollars. Not wanting thumping bass, just fuller sound. No experience with subwoofers so, ,,,,
Thanks for your input.