Subwoofer recommendation and wisdom

So I have a relatively difficult room 19' x 19' x 9'. I have made a bunch of sound absorbing panels which made a massive improvement. I have worked on speaker placement and landed on the 5/8 ratio......5x from the back wall, 8x from the side walls. I use Harbeth 30.1 speakers on appropriate stands, driven by a Luxman L505 xII integrated. I am very pleased with the sound but sometimes wish I had a bit more foundation on the bottom end. I will listen mostly to classic rock and jazz. I would like to consider adding a sub, but not sure where to start. I don't want sledgehammer bass, I just want a nice, blended bottom end on my music. I think that is one of the only improvements I can reasonably and cost effectively make. Looking for recommendations on a sealed sub that would have a good chance of integrating well in my room.

My preference would be for a single sub solution. Thanks