It would make a lot of sense that you first determine how low your speakers go in your room. I would get an SPL meter and measure. When you find out what your cut off point is, then you can make a better decision as to what you need. I don't think you can do this by guessing. SPL meter will also help you integrate your sub much better than any ears could do. I hope this helps.
Subwoofer recommendation
Von Schweikert Vr-8's
There are giant, They are rated to 20hz but due to room I don't think they get there.
Looking ta around 2K used
Something that gets out of the way unless I want it to and can rock the house.
Looking at Rel t9X?? Rythmic, ect.
Want to do this once...