Distributed Bass Array configuration

Lastly, your main speakers will sound much better if you high pass them at 80 than at 40 Hz.

Depends on the room, main, sub and processing available. My mains are -3dB @ 31Hz measured pseudo-anechoic. ≈40Hz integrates better than 80Hz.

When I used Spica TC-50, ≈90Hz.

The clarity obtained by LF attenuation in the mains with good subs should not be ignored:

1. Increased apparent amplifier power
2. Reduced voice coil heating which preserves XO frequency to the mid driver.
3. Reduced IM & THD in the woofer
4. Reduced current demand and its consequent mid & tweet modulation.

Tim ( noble101), yes, it does get better as you add subs. I couldn’t believe how much improvement there was adding a third sub after already getting the improvement of adding a second. My front subs have to go to the inside of the mains because there is an opening to the entryway on the right and an opening to a hallway and a sitting room beyond that on the left. Open floor plan and basically no corners either in the front. So that may be one reason for a dramatic improvement of the third. Don’t know if I’ll add a fourth.

What measurements establish the “improved bass”?  Improved in what way?

Just  wondering.  Is it just more? More  extended?  Flatter response ?  Different frequency curve?    What exactly is different than prior?

I have two twin-18” subs; run in stereo. 

MAPMAN -
It definitely does not get better because there is more. These guys you see on YouTube ( some considered influencers ) with multiple subs primarily in their theater systems are going for more bass and sound pressure. I guess if you just want to impress people by making their bowels move and shock the crap out of them ( pun 👍 ) go ahead and get your kicks that way.
You’re not getting more  bass if you have set things correctly. You can get more if you don't mind it being out of balance with the upper end but that’s not the way I listen to music or movies for that matter.
What has happened for me ( doesn’t mean it’s going to be exactly the same for anyone else in their room/configuration ) is that with two subs there was better blending ( horizontally ) without the perceived location issue derived from one sub. But, then the bass ( especially kick drums ) were overly emphasized and got just so centered that it was almost the only spot many of the lower frequencies were being placed. Unfortunately I have to set my front subs to the inside of my mains ( no choice ) so that influences the issue.
But when I added a third sub, wow!! Game changer and now I’m elated. The low frequencies are completely balanced in every way with the rest of the music ( frequencies ). There is absolutely no sensation that there are subwoofers ( external speakers from the mains ) and the bass is just part of the music ( heavenly ). The bass is still centered ( that’s the way it was mixed by the engineer ) but it spreads further horizontally and very naturally ( as I would perceive natural ). It fills the listening area ( I’m not saying there is bass all over the room ), making  it smooth with no overemphasis anywhere. I feel the subharmonics without it feeling like they are ”hitting” me ( even with movies ). Not that it’s due to multiple subs but I’m relaying that you don’t get that type of issue because you have added multiple subs. I think there can be that perception from the hype of the bass centric crowd.
What amazes me is that the third sub is less than six feet ( behind me ) from my listening position and I don’t “feel” it even at higher volumes, it blends sonically invisible.
None of this would be possible to achieve setting up by ear ( well maybe with a couple of years of trial and error ). It’s only been possible with the implementation of the MiniDSP HD 2x4 and REW.  The full use of REW with its eq section ( room correction with parametric eq that is similar to Dirac ). Beyond that the MiniDSP offers delay options so after the REW eq you can make tone sweeps and see which delays ( or lack thereof ) on each sub is most beneficial ( or not ). This is the best $315 I’ve ever spent!! Sometimes technology truly is your friend.
Now there is quite a learning curve and definitely trial and error but it is more than worth it!! Some may consider themselves to already have achieved this but I’m confident that if you implemented the MiniDSP you would achieve even further benefits and results.