Atelier Rullit field coil driver

The compact Gallo TR3 subs are "self powered" and would just need to be adjusted (on the back) to the horns.

They are a typical powered sub with an onboard crossover network.

Two would be required as bass becomes noticeably directional above 60 Hz, or so (one sub per channel located close to or below the horn).

Figured it’s a realistic approach considering the room size/budget.

I’ve tried the TR1 and TR2 subs with some of my single drivers and they mated/blended quite well crossed over @ 100Hz.

Plus the 250 horns are not that large/deep.

PS: You need to use the low level inputs on the subs for the crossover frequency adjustment to be active.

https://galloacoustics.com/product/tr-3d/

DeKay

You might consider the PureAudioProject open baffle speakers.  Their Quintet models feature four 15" woofers, with various modules for midrange/tweeter that fits in the middle.  These modules include a horn option, a coaxial driver option and a couple full range driver options, including a Voxativ field coil option.  They have a smaller speaker called the Trio, which features twin 15" woofers and the same array of midrange/tweeter modules.  Another great sounding open baffle system is made by Cinnamon Audio--it uses an AMT tweeter and active, DSP-controlled woofers. To me, these are very good sounding systems. 

Songer Audio makes a single driver field coil system that is also very good sounding.  Another speaker I really like is the Companion model from Charney Audio.  This is a single driver, backloaded horn system that is impressively full range for a single driver system.  The offer a variety of choices for that single driver, including models from AER and Voxativ. 

My favorite speakers are not regular commercial products; they are custom made horn-based systems from Deja Vu Audio (Vienna Virginia) that utilize some very old midrange compression drivers and horns as well as modern custom made woofers and tweeters.  They make speakers from about $10k on up to who-knows-where.  The better models use drivers from the 1930's-1960's, and horns from the same era.  At their shop, they even have a massive Western Electric 16A horn with vintage drivers and modern woofers and tweeters (the horn is three years shy of being 100 years old).  Fun stuff.  If you can make it to the Washington DC area, this is the place to visit for out of the ordinary gear (they also make their own tube amps, linestages, phono stages and DACs).

Thank you for all the kind recommendations, although I might accentuate that I'm looking for a suitable cabinet idea for the Rullit field coil driver.

The appropriate cabinet will depend on the specific design of the driver.  The manufacturer can tell you if it is designed for open baffle, sealed cabinet or bass reflex and the appropriate cabinet volume.  Drivers don't behave ideally in different types of systems, although manufacturers may make different models for different uses.  I heard that driver in two different systems, both were open baffle.  

Also, a local speaker builder who uses field coils in some of his builds has encountered problems with overheating when field coil drivers were used in closed cabinets.  That is probably another thing to ask the manufacturer about.  Audio Note makes a seal cabinet speaker (AN-E) with both the woofer and tweeter being field coil drivers, but, they claimed that keeping heat down to acceptable levels was a major engineering challenge.

@apollinaire head over to WBF and you'll see a great thread about Rullits. A friend of mine has the 9 inch Super Aero, and i've heard them in an open baffle and they were superb. He's now working on horn loading them.