You might want to consider a mighty 8” wonder from B&W, the ASW608. In the video below a gentleman tells of how well the ASW608 blended with his Cornwalls.
I don’t have Cornwalls but I decided to try them in my system. I added two and they are excellent. B&W provides some very good basic DSP options on it for ease of blending with your speakers. They are small, unobtrusive and made well. For an idea of how they look check out my system pictures.
Adding the subs has been the best thing I have done for my system.
Cheers,
Scott