Will I benefit from a subwoofer with 20Hz speakers?

This comment displays a lack of understanding. If nodes developed randomly it would be impossible to treat them because you have a moving target. Any given speaker in room will set up a consistent pattern if nothing is added or removed and the doors,

Thanks for taking my comment out of context, @lemonhaze

My meaning was that most audiophiles should not take the -3 dB point of a speaker and think that it means much once it is in a room. While the physics of room modes is understood, the final -3 dB point of a speaker in a room is not something anyone can gauge well by back of the envelope calculations. That is what I meant by "random" and why I encourage measurement as a much better place to start than speaker specs.

Room mode simulators like the great one from AM Acoustics are fantastic tools which should be leveraged when considering room treatment, and I encourage it’s use as well.

https://amcoustics.com/tools/amroc

This is really a side comment and doesn't apply to main speakers vs subwoofer.  I have found that frequency response down to 20 Hz is not necessarily that important, especially for music.  There are definitely recordings which you can get that do go down to 20 Hz, but they are a rare item.  If you really look at how most people respond to bass in music, the most important area is the 40-70 Hz area.  That is where we feel and perceive most of the visceral impact of bass.  The low 20 Hz rumble is nice, but if you have a hole or bass null at 50-60 hz, you will feel a significant lack of bass.

Oh, one more note.  I would be very careful about using Dirac to eq the response of these speakers flat down to 20 Hz.  The Cabass is stated to having response down to 41 Hz, which is definitely possible with the right 6.5" drivers.  However, past a certain point, the driver response is radically reduced the closer it gets to 20 Hz.  While those drivers -could- be reproducing a 20 Hz signal, the actual sound level can be something like -20db or -30db.  This means that Dirac is putting a +20db eq for the 20Hz point and will create a massive over-excursion of the speaker cone if you do actually hit a sound with this frequency.  This will definitely cause the woofers to either peak and clip - and can damage your speakers.

 IMHO don't waste your money. 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, did you not read the thread? If you did you would have seen my short explanation and @atmasphere's clear and informative take on the virtues of multi-subs.

Perhaps you do the OP and yourself a disservice with your ill-informed dismissive reply.