I have large speakers with 15" woofers that are spec'd to 30 hertz. When I measure my in-room response I have strong output to 20 hertz, the lower limit of my analyzer. I would still like to have subs but space and monetary concerns have kept me back.
A long time ago my speakers were Dahlquist 30i's which were considered full range but I felt the low end was too polite and I picked up a 15" Velodyne sub just to see what a sub would do in my system. As expected, there was no increase in slam but the low end reached much lower, which I did like. I picked up a second sub mostly because I could tell where the sub was when listening and I wanted to even out the output.
What I didn't expect was the change in soundstage on live recordings. I attended a lot of large venue rock shows back then and with the subs I had a lot more of a feeling of the venue space. I reasoned that in a large hall or stadium there was a whole lot of subliminal sound that I could now hear-- noise produced by the shuffling of thousands of feet, people talking and eating, and more stuff that was producing low frequency sound that I just hadn't been aware of in the recording until I added the subs. It added a whole new dimension in my room, and made those live shows much more realistic.
The setup I have now plumbs the depths pretty well but not like those subs did, and I miss the environment they provided. But to get what I think I would like would require large subs that reach real low with high output, and a pair of such would be physically imposing and lots of $ so I get by with what I have....
Bill