Before you buy any sub read this first. https://soundoctor.com/whitepapers/subs.htm
In my opinion if you are not listening to loud bass driven rock of some type, the addition of a subwoofer to a full range speaker will not make a big enough impact to justify the expense. Also keep in mind that the ear is less sensitive to low bass so if you want to experience 40 Hz at the same perceived loudness as 1k Hz. You need to be able to reproduce the 40 Hz. signal approximately 15 to 30 db louder the the 1k Hz signal depending on the 1k Hz overall volume level. That takes a lot of amplifier power.
Having said all of that I own four subs and I would never do it any other way. I own REL, Rythmik and SVS. The only thing REL does better than the other two is to take more money out of your wallet, otherwise very little difference between them except the REL can be a little more difficult to integrate well. I feel the Rythmik has a little better sound than the others but they seem to have slightly less power, so I would err on getting a larger size rather than the smaller. SVS in my experience has the better customer service.