Pure Audio Project open baffle speakers

I'm sure they go very loud which is impressive but it does not mean the sound quality is good. In their favor is that the woofers become very directional along with the horn which is directional. This would limit room interaction nicely. The cross over point here is critical. They have to keep that horn out of trouble but you want to get away from those woofers as soon as you can. The very low bass is going to be very confused. I would cross over to a sealed sub at 100 Hz.

Williwonka, I am thinking about wavelengths, 100 Hz being about 10 feet.
As you get to the bottom of a drivers response curve it becomes more and more omnidirectional or conversely as you go up the frequency response curve the driver becomes more directional. The most important factor here is the size of the driver. As the woofer goes down it's response curve the front wave cancels out the back wave. When you put the driver in a baffle board you can separate the front wave from the back wave down to a certain frequency depending on the size of the baffle. How is a 24 inch baffle board going to separate wavelengths over two feet? It can't. In open air you get a predictable situation with the response curve falling off as the wavelength exceeds the size of the baffle board. In a room the situation becomes very complex and unpredictable with waves bouncing off walls and interacting with each other in very complex ways. The result is very unpredictable response in the bass, an unhappy situation. I can only guess but I think this passes with many people because they have not ever heard a system that goes flat down to 18 Hz.
This is not easy to achieve. Many very expensive systems do not do bass well and in many instances it is not the system but the room. You can easily get 10 dB aberrations in response in your average listening room. 
With an unbaffled  driver this situation just gets worse. 
Some people deal with this using a "swarm" system where they randomly place multiple drivers throughout the room. I deal with it by using a bass linear array and room control. You could never manage an unbaffled subwoofer system with room control. It is impossible to increase the volume of a frequency that is desperately trying to cancel itself. The response has to be +- 5 dB for room control to work well.  

I forgot to mention, I use full range ESLs so I am handily in the open baffle club. At higher frequencies it turns out to be an advantage in your average room as you have more control over dispersion and can limit reflections. I do recommend placing acoustic tiles on the wall directly behind the speakers and crossing to subwoofers no lower than 100 Hz. I cross over at 125 Hz and with eyes closed you would never know there were subs in the system. Also many tweeters have closed backs. If this is the case you'll want to add a rear firing tweeter.