Will I benefit from a subwoofer with 20Hz speakers?

It’s good you’re using Dirac in that room otherwise your system would be unlistenable because of the bass amplitude.

But I’m not sure there is a need for using Dirac above 500hz.

And, don’t trust the Dirac curve too much. If you measure your system as @djones says you will almost certainly get something else. And what you measure right now may also be distortion to a great extent. The freq. curve is just the beginning.

I use some Dirac for the bass yes, but then apply Roon eq according to my hearing.

I’m not of the opinion that Dirac and similar dsp tools gives a too lean bass. Quite the opposite. Don’t trust the smooth calibration curve.

I also use a phenomenal sub to my true 20hz speakers. For the possibility to show off when someone wants more bass :-) but also for HT use and for the fact that some recordings really need a push down low.

More subs will also give you a better distributed bass in your room, some says. I’m not so sure that will help a lot. You’ll still have peaks and dips.

@dmilev73 My main speakers are flat to 20Hz. But at the listening chair I have a cancellation, so no bass at that location, while nearly everywhere else in the room the bass is fine.

So I added a pair of Swarm subs. One is to my left, immediately against the wall (I actually have its driver facing the wall about 2" away and the other is behind me and to my right, set up the same way (the Swarm subs are designed to take advantage of the room boundary effect and so are meant to be directly against the wall; most subs are not). Because of the asymmetrical placement, the subs are able to break up the standing waves caused by the room itself. You can turn the subwoofer amp on and off; the difference is dramatic. Off- no bass, on- bass, off- no bass, on- bass. Obviously I leave the sub amplifier on.

So yes, additional subs can help out speakers that go to 20Hz all on their own.

Here are some facts to understand how this works:

1) the ear can’t know the bass note is there until the entire waveform has passed by it.

2) at 80Hz the waveform is 14 feet long; so by the time your ear knows its there, the waveform might already be bouncing off the wall behind you.

3) the ear can’t tell the frequency until a few iterations have passed. By this time the bass in the room is 100% reverberant.

4) because of 3), a mono bass signal can be used for the subs.

5) if you keep the subs from having any output above about 80Hz, they won’t attract attention to themselves.

6) the asymmetrical placement is important for breaking up standing waves.

More subs will also give you a better distributed bass in your room, some says. I’m not so sure that will help a lot. You’ll still have peaks and dips.

Yes, you still have peaks and dips but they are much smaller and are all over the room. Because there are so many more and they are less than 1/3rd octave apart the ear won’t acknowledge them. So it helps a lot!

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.