Add a subwoofer?

james633,

This is what I use to convert my speaker level to balance for use with my JL Subs. The JL subs have an excellent internal calibration for the room placement.

SP-2SX | Jensen Transformers (jensen-transformers.com)

ozzy

If you just want a little more bass, but don't have much money, how about adding a simple Schiit Lokius to add a bit more umph down low?  2 of the lowest bands are at 20Hz and 120Hz so feathering in those controls to taste should give you what you want.

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

https://www.youtube.com/watch?v=n7YtBFD473U&t=3s

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.