Will I benefit from a subwoofer with 20Hz speakers?

"...most of the energy that drives your system will be shunted to drive the subs, and your amp driving the main speakers will be starved and get only the meager leftovers that the sub graciously allows."

That should not the case if the sub is active.  The main speakers will not be affected much but if you have high-pass filter (HPF) in between, then it will leave more headroom for the amplifier to dirve only the mid and high and therefore more efficiently.  In fact, I found, without HPF, the integration of sub with the main speakers wont be as good because the mid-bass was often boosted a bit hig to my taste.

I will play the devils advocate here. While adding a sub has the potential to add LF quantity and / or quality, it will invariably take away from the signal strength going to your main amp / speakers. That is, you need to make compromises at the low signal level, which might be quite steep especially when the subs plate amp with super low input impedance guzzles the current away from a tube power amplifier with high input impedance.

No, this isn't a thing.  With preamps easily able to under 1 kOhm loads, and amps  presenting 25k Ohms and higher loads there is a barely measureable change to the preamp output at the same setting.  Do the math.  Assume a perfect voltage source with a 1 kHz output impedance.  Takes a lot of 25kOhm amps to cause a change.

@dmilev73  Yes you will definitely benefit from a sub or 2 or 3  When this is suggested people think of being overpowered by thunderous bass which if properly implemented is not that at all. Think of the subs as tuning devices. An enlightened poster mentioned a SWARM and I encourage you to read up on this. (aka DBA)

But the thing is that rooms and speakers are random.

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, windows and drapes are left as they were when first measured. Come back in a week or a month and the room will measure exactly the same.  Nothing random about it.

@lanx0003, Who told you to toe-in your speakers to cross in front of you? This relies on the proximity method and works with speakers that use constant directivity waveguides and a power response that is even.  A 180 degree waveguide, like the ubiquitous dome tweeter will not work which is probably what you have. A CD waveguide toed-in like you tried provides great imaging and a nice w-i-d-e  listening area.

@lemonhaze Erik (@erik_squires) has advised so.  So let me get this right since it is kind of interesting.  You mean if you had CD waveguides tweeter like horn, you will benifit from toe-in to cross in front of you as it will provide great imaging and wide soundstage AS COMPARED TO straight or slight toe-in, right?  I wonder what is the "theory" behind it but will definitely give it a try when I get or audition speakers with CD waveguide tweeter someday.  It may be my biased preconception but I never had an interest for horn speakers as it are a bit too bright for my taste.  But maybe some other less "agressive" (not sure if this is a right description) CD waveguide tweeter speakers will serve me better.

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.