"why it's not so easy to turn a mini into a 3-way"
Copyright Troels Gravesen
       

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I often have the question if one of my smaller minis can't be made into a 3-way from using 6-10" bass drivers by adding an e.g. simple 1st order high-pass filter to the mini and "some" low-pass filter to the bass driver. What's it going to look like?

There are a number of problems in this and I'll try to list a few.

1.
First of all a small mini usually makes 82-83 dB/2.8 volt, where the suggested bass driver may produce significantly higher output, like 86-90 dB/2.8 volts, thus a major discrepancy in sensitivity, and for a number of reasons we just never want to reduce a bass driver's output by series resistors. I won't go into detail, but it's a no-no.
The mini is usually tuned to make some bass output down to 60-70 Hz depending on driver size, i.e. cone area, the mass of moving parts, magnet size, voice coil impedance, etc.
The small 4-5" bass driver may have a sensitivity around 85-87 dB/2.8 volts in the 500-1500 Hz region, but this doesn't help if the bass region can't produce the same output, thus we add a series coil (we equalise) to tame the midrange and to compensate for baffle step loss, and we end up with a system sensitivity around 82-83 dB/2.8V.
The same driver may very well be suitable as a midrange driver in true 3-way system, maybe even tuned to 90 dB/2.8V system sensitivity despite only having 86-87 dB sensitivity in mid/upper-mid, but more on this later.
Let's take a look at
Eekels' Minis.

Above the predicted response from LspCAD. System sensitivity around 83 dB/2.8V. Now, if we add a simple 1st order filter @ e.g. 300 Hz (100 uF for 5 ohms impedance at 300 Hz) we get this:

This does look a bit weird making a major bump at 800 Hz and anything but an ideal lower roll-off profile. Adding a 2nd order filter at 300 Hz looks even worse:

To cut a long story short, it doesn't work. Making a crossover for a 3-way system is an integrated process taking into account all drivers at once and we can't just add passive high-pass filters to an existing filter without seriously compromising performance.

2.
Second option is using an electronic crossover and no objections here, but the mini will still only produce 82-83 dB/2.8 volts and it's never going to be a "big" system. We're into a sub-sat situation and we may like the mini for what it does so well, but it will never produce the kind of sound level we expect from a large speaker system.
And I have to emphasize here that buying a big amplifier doesn't turn a low-efficiency mini into a big speaker. I often have that question too. Even the
TJL3-way isn't particularly sensitive - or big - and a 200 watts amplifier will never make this speaker into a really big speaker due to the cone size of the bass driver. Look at membrane size when it comes to acoustic output. Read more here.

3.
Next we pick an 8" bass driver, e.g. SEAS W22NY001, and we start modelling and maybe we get something like this:

Now this is a completely different story. The W22NY001 makes a decent midrange and works all the way up to 800 Hz. The W12, now working as an upper mid driver, only covers the 800-3200 Hz range. No system optimisation applied here. Just an example.
What we see is a small 4" midrange being used in a larger system tuned to 90 dB! Some of the upper mid actually comes from both the bass driver and tweeter due to overlap. You can see the red graph being below summed response (black).
If we wanted a lower point of crossover between the bass and mid, we had to pick a larger and more sensitive midrange, e.g. M15CH002. It may look like this:

The M15CH002 is a bigger speaker with higher sensitivity and can handle most of the midrange and we can move point of crossover down to 350 Hz or even lover. This is just not possible with the small W12 driver due to its low sensitivity.

So, if you enjoy your minis and think of keeping the qualities of these minis, buy a 24 dB/octave electronic crossover plus a second amp and experiment with points of crossover between 80-160 Hz. This will relieve the mini of having to pump bass and you can play a little louder.
If you buy a sub with integrated amp and in-built low-pass crossover and have the minis run fullrange, the minis will still have to cope with the bass signal and not be able to play any louder than before.

Turning the drivers of a your mini into a 3-way system is starting all over and we need to do what's described here.