Will I benefit from a subwoofer with 20Hz speakers?

@lanx0003, a constant directivity (CD) waveguide and a horn look about the same but behave differently. Generally speaking waveguides do not exhibit any of the annoying traits that gave horns a bad name. A CD waveguide in addition to allowing a wide sweet spot also has extremely low distortion and seemingly unlimited dynamics whilst also providing a believable stage.

They are not bright if correctly implemented and I find superior to any dome tweeter. In a domestic situation they are virtually indestructible.

As I've mentioned before: Pink Floyd don't play through dome tweeters 😎

There is very little musical content in that octave 20-40Hz and in my experience having too much bass becomes annoying after a while. 

@bishop148 The point is not more bass! The point is to get the bass right. If the bass is deficient, the ear will perceive the system as being tilted to the highs; if there is too much bass the ear will perceive the system as being muffled in the highs.

If you listen at mid volumes (or your room is large) then a sub should be of some benefit,  If you listen some of the time at low volumes then a sub will always be of great benefit.  You can do the equalising thing (early amps always had a 'loudness' button!) but a good quality sub is the better way.

Justification for subs: The best place for imaging is never the best place for bass™

Op: I am not in any way saying you should not get a sub.

Properly configured a sub can be glorious.  I just want you to maximize your current room and system first.