Creating endgame 2-way speakers

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From the RAAL 70-20XR datasheet, it looks like a LR4 acoustic slope crossover at 2,600Hz is produced when the 89db sensitivity terminal is selected, and the 12.6 ohm terminal is selected, and a 3.3uF series capacitor is used. If this is correct, then 2,600Hz LR4 would be a good first build with the C-Quenze 18H52 17 06 SD to study cone beaming and cone breakup.

How difficult is it to create alternative frequency LR4 acoustic slope crossovers?

Your interpretation is correct, use the 89dB tap with a 3.3uF cap to get 2.6K cross.

The C-Quenze 18H52 17 06 SD is a really, really, really nice midrange.

You really want a cleverly designed sealed cabinet to experience the really detailed transients.

So, you want to construct a RAAL + 18H52 cabinet which can easily convert between ported or sealed.
----no front slot port
----rear round port which can easily get plugged

Build a cabinet with ~0.6cuft empty.
-PORTED: 0.5cuft port tuned to 51Hz, -F3= 53Hz => 2" diamater port, 5" long, round both ends, 1350Hz port resonance
-SEALED: 0.5cuft FSC=74Hz, Qtc= 0.57, -F3=94Hz

I would.....
Use physical time alignment between the RAAL and 18H52.
Use 2" to 3" edge rounds or bevels to reduce diffraction.
Use LR4/LR4 crossovers at 2300Hz.

Another option is since the 70-20 with a cap will net you an LR4 acoustic crossover, you can use a second order low pass filter on the 18H. This way you take advantage of group delay and don’t need physical time alignment, which it looks like the Vapor speaker is using. I think for an 18cm driver, you should look at a 20-25L enclosure, vented with a 2” by 7” rounded port. I agree with LineSource that using a pluggable port is a great idea and here’s why:

I don’t know if you’re familiar with Jeff Bagby, but he is a well respected designer of 2 way Monitors (see Kairos, Continuum) and he created a sort of “universal woofer module”. I can link you to his paper where describes a 10” woofer in a vented cabinet that a 2 way monitor can be mounted to, in order to create a true 3 way tower. This way you can build a 2 way now, and when space permits, build a woofer add-on and get deep excursion into the 20Hz range. Having a pluggable port will be great so you can take advantage of the transient capabilities of the 18H and just vent the big woofer.

On that note, I would strongly also consider using an Accuton 6.5” ceramic driver, the C173-6-096E in your two-way and eventual three way system. Many reviews have praised its midrange detail abilities, Vapor says it has greater detail retrieval, and I personally have been a huge fan of it in the systems I have heard. That being said, it cannot dig as deep as the 18H on paper, and I have never heard the 18H so perhaps other folks here can chime in. I think a RAAL tweeter + ceramic midrange + Per skaaning big woofer might just be your “endgame”.

As I said, I’ve never heard the 18H so it may be as good as the C173 just using different materials. Either way, build your two way confidently today and look to turn it into a three way tomorrow. The crossover design won’t be easy and you’ll have to make sure the cabinets are well designed and damped, but you will have an incredible reference system to listen to for years to come.