Will I benefit from a subwoofer with 20Hz speakers?

@erik_squires Again, if you do a life music recording out of the 40Hz-20kHz speakers and the flat 20Hz-20kHz treated room, your claim will be much more convincing.  Post it on Youtube and you will be famous.  Why not?  Your audiences should not be limited to audiogoners.

@erik_squires You owe nothing to anyone or me. Your voluntarily service to the audiogoner in the community is beneficial as I have complimented previously. However, when you make such a strong/powerful claim and try to maintain the credentials when others challenge, the burden of proof is on you. It is a reasonable doubt that what you have measured is one thing but what is actually heard is another. It is your decision to do what I have suggested, which is another way to disprove something that I and a circle of friends of mine don’t believe achievable. We all do not believe will be so "lucky" as you are...

Oh, BTW, last time your advise toeing speaking in front of MLP and I reply with the shrinkage of SS as a result, you ignore too.  Is it another way of yours admiting something you have advised is wrong?  Ignore again.  I don't care but just wish get some mindful message across so others will not follow the wrong footstep.

@erik_squires I really would like to take your words for it and tune my 44hz and 39 hz (+-3db) bookshelfs down to 20hz (flat like you have shown or -3db is fine). This really sounds a bit too good to be true to be honest. But before we invest further for expensive GLK panels and treat my room as you did, is it possible for you to "demonstrate" the sound reproduction from your room by some real-life recordings? I think, with all the measurement equipments you have for DSP, doing a quality recording is not that hard, right? Are you able/willing to do that for the good of the group?

https://speakermakersjourney.blogspot.com/2021/12/room-speakers-eq.html

That was what I was saying "it is too good to be true" (without the aid from sub). I believe you know quite well that, in order to boost 6dB, you need to double the volume. So I wonder why do you even make fun 😀 of people when they question that as below. It is simply not doable either practically or in theory. That also lead to my next question in your post where you mentioned: "... Take my modest speakers. They use 6.5" woofers and a ported cabinet. Would you guess they went down to 20 Hz? They do..." How you achieve a flat response down to 20 Hz there? What is the frequency rating for your modest speakers (Squires SNR-1?)?

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?

The words of those who don’t have a lot of practical😀 experience.

Eric, I do not wish to prove you wrong.  In fact, since I know your existence in the forum and read several post of yours, I always respect and consequently place more weight on what you have advised or recommended to improve my system.  So far, except last time you said to toe-in speakers and cross in front of you that I found by doing so shrinks the width of soundstage, everything else is spot on.  Unfortunately, this is another incidence that I was (and I believe some others were) not convinced.  Let me explain by telling you my experiment first.

I use foobar2000 as a player and under the "View" "DSP" you should be able to find "Equalizer."  The reason I specifically refer to this particular EQ tool in foobar2000 is because I found this is probably the best EQ among all the players I have experience with.  I trust the fidelity of this tool.  The lowest frequency this EQ can go is 55Hz and you could adjust all bandwidth up and down by 20dB.  I play a track entitled Keep A Secret by The Whitest Boy Alive from their album Rules which begins with heavy weighty drum beats.  Then I gradually ramp up the 50Hz-156Hz bandwidths in a linear fashion, i.e., starts with +1dB at 156Hz, +2dB at 110Hz, +3dB at 77Hz and +4dB at 55Hz and ramp up by a certain level while keeping the slope linear.  When ramping up the 55Hz to roughly halfway (+10dB), I have already hear distorted bass notes.  I stopped with the worry ruinning my speakers.

The reason I did this experiment is because I suspect that what you and the other forumer have measured in your room as shown in SPL down to 20Hz might be some highly distorted noise, not the pleasant musical notes any more.  That is also why I am asking you to provide some quality recording to the group demonstrating what we are going to hear when you tune the speakers from the rated frequency (say +40Hz) down to 20Hz.  I could be wrong and I wish I am wrong.  I wish the tuning / DSP is successful out of the room treatment so we could all enjoy/benefit from this experiment.  I hope I make my point this time. 

@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.

"...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.