Music in the 40-80 Hz range

More LF sources is better. Each LF source creates its own room modes depending on the frequency and location of the source. The few big modes you get from one sub won't sound nearly as smooth, fast or good as the many small modes you'll get from three or more sources. Try it and you'll see.

There's actually quite a lot of music happening way down low, its just that its so hard to reproduce well. Until I got my four subs I had no idea how much.

The answer is almost everything will be grounded in this range.  Pretty much anything with the double bass will cover this. Try a good jazz double bass and vocals track to test how well you are getting integration

https://www.studybass.com/gear/bass-tone-and-eq/bass-frequency-range/

@dbphd, 

There's very little music in that range, (most of it lies further up in the midrange) but without that range some of your playback will sound very thin and weak.

Apparently, what most of us consider as good strong bass is far closer to 60Hz than it is to 30Hz!

So if your speakers are flat down to 40Hz, then that's good enough. If not, then a good sub could be of interest if you don't mind the extra cost and complexity.

In your case, if we can assume that your subs are better at handling <80Hz frequencies than your Kef's, then I would leave things as you currently have them.

Your Ref 1s should then benefit from having all that tough low freq workload taken off their hands.

Why not try some reggae at healthy volumes and see what you think?

If there’s very little music below 80 Hz and our hearing doesn’t go above 10 kHz I guess we can all go back to boom boxes. What a relief! 🤗

Your ears cannot make the determination? You would let consensus dictate what you like to hear? 

I have plenty of reference tracks I use for bass. Key here is the mastering... the bass sound be strong and impactful and you need a mix of songs to demo bass.
Yeezy - Put on = there are very strong fundementals in this track, one at 34hz, and another at 28 hz. The bass should rumble in your tummy if the speaker or subwoofer is doing its job.
Keb Mo - I don't know - great example of masterfully mastered guitar bass. This will kick you if your system is setup right. very strong bass from mid 30s to 100hz.

I wanted to give a quick mention to Lenny Kravits American Woman, and a few parts in Dire Straits Private Investigations. both have kickdrums that should smack you right in the face.
Nils Lofgren bass and drum intro. Very good texture on this recording so if your system can resolve well this song should sound very good.
Grace Jones - Hurricane Dub this is a pretty good one to stress at high SPL.
And last but not least you need to play Chemcial Brothers under the influence to test any respectable systems bass performance.

schroeder, your post was needlessly contentious and without value, and "let consensus dictate what you like to hear" a non sequitur.  

I wanted a fully balanced passive filter because it goes between an Ayre KX-5/20 preamp and VX-5/20 amp, but that filter has fixed parameters.  I was second guessing myself about whether I should have specified a 50 rather than 80 HZ crossover or perhaps continued the low-pass-only configuration.  Further listening to a wide range of material since my original post suggests the filter is working well.

I "like to hear" baroque and jazz, and the occasional large orchestration.  The source tends to be Roon via ethernet to a QX-5/20 digital hub.