Will I benefit from a subwoofer with 20Hz speakers?

Well that's not bad.  I think you left the bass a little lean. 

Between 20 Hz and 100 Hz add a low shelf of about 4 dB.

from 100 Hz to 16 kHz go for a steeper slope, so 16 Khz is about -5 dB relative to 500 Hz.

Yes, subwoofers can be glorious but your problem right now is room balance.

Also, if you are doing this all without any room treatments it's not going to sound as good as if you worked together with them, especially in the bass.

Assuming 18ft between front and back wall your room will amplify 1125/36=31.25Hz and multiples (62.5Hz, 93.75, 125Hz etc.).  31.25Hz is alway welcome, but multiples require room treatments.

A good Subwoofer will be far more articulate and controlled and far better punch such as Svs 4000SB  no contest for just over $3 k a pair if you get them on sale from the outlet. The Cabase no way it get 20 HZ with impact reaching 20 hz for a millisecond won’t cut it ,my brother has bigger Wilsons and still has these same 

Svs subs  tuned with their excellent App,Free  REW bass program ,and you buy a DSP Mike ,after learning and taking a few readings you sent to SVS  and they tell you exactly where to set the parametric eq on the app to truly flatten room bass nulls and tune the Bass to your room , night and day better Bass, Svs pays shipping ,a 45 day trial 

and even return shipping $100% refund if not happy and 5 year transferrable warranty ,a great company to do business with.

Yes you need subwoofer without doubt there be shadow.

there's no way speakers that are rated at 41 HZ are going to go down to 20 HZ in your room what kind of measurements are you using?