I also have a small listening space and was concerned that a large sub would completely overpower the room and make integration all the more difficult. After reading numerous professional and user reviews, I ended up buying a small Rel T/5i sub that paired quite nicely with the Kef LS50s. The T/5i has an 8" driver in a sealed enclosure. It has Low Level (RCA) and High Level (Speaker) inputs. Even comes with a Speakon cable for High Level use. Definitely include the Rel T/5i on your short list.
https://rel.net/shop/powered-subwoofers/serie-ti/t5i/?gclid=Cj0KCQjw7sz6BRDYARIsAPHzrNK8NcjURmxJVl9G...
https://rel.net/shop/powered-subwoofers/serie-ti/t5i/?gclid=Cj0KCQjw7sz6BRDYARIsAPHzrNK8NcjURmxJVl9G...