Atelier Rullit field coil driver

I have heard the Atelier Rullit field coil driver in Trehause open baffle speakers.  I cannot attribute specific qualities to that driver, but, I can say that these were not among my favorite open baffle speakers; the sound was a little thin and rough.  Not a bad speaker, but, not the best in my opinion.  Among the better field coil wide range driver systems I've heard is the Songer Audio model; the sound was rich and free of the kind of harshness or peakyness that I hear with some full-range driver systems yet it retained the dynamic qualities of such drivers.  The Audio Note two-way AN-E speakers with field coil drivers sounds good too, and it is not a fiddly thing (one does not have to adjust voltage) and it appears to overcome a common problem with field coil speakers in an enclosed box, which is overheating.

I've heard many custom-made systems employing field coil full range drivers and field coil midrange compression drivers for horn-based systems.  As mentioned above, overheating is an issue with enclosed cabinet systems, and for some systems, the ideal sound requires a bit of adjusting of the voltage as the system warms up.  With the compression drivers, I heard a pretty substantial difference in sound based on the power supply feeding the drivers.  I was shocked to find that the power supply made such a big difference; the tube-based supply sounded better than a solid state supply.

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).

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.