There's the deal.
OBs/dipoles need a lot of equalization. About the only successful passive OB i've heard of is Jeff Bagby's Archos, and the thing to note there is that the woofer section is a box / monopole and the drivers used are both very high expensive and very high efficiency.
Passive crossovers can attenuate peaks, but in order to "raise" a dip with a passive crossover, you unrealistically need to attenuate the entire rest of the response.
Now here's the deal with a dipole speaker:
http://www.musicanddesign.com/Dipoles_and_open_baffles.html
as you can see from reading that, dipoles require
boost in order to produce the same output that a monopole would. If it starts rolling off at 6db/octave below 300hz for example, that means where a driver might be 0db down at 75hz in a sealed box and, it'll already be 12db down at 75hz on an open baffle. 12db of boost to return it to flat - is a LOT and that's one part of why OBs lack amp headroom.
Now compound that with the fact that any driver in free air or a sealed box rolls off at 12db / octave, so your bass that's already acoustically 12db down at the knee, is now going to roll off at 18db/octave below that knee.
So dipole bass is not very efficient. You need world class drivers and world class amplification.
There's also the reality of driver excursion. Even though you can boost the "12db down 2 octaves below rolloff" as I described above, the boost doesn't mean you're returning the driver motion to where it would have been in a box. It actually has to
move more to produce the same output, because of how much cancellation is going on. As a test, run a 30hz tone through a woofer in free air, and run that same tone in a box. One will be louder, even if the driver motion is the same.
That's one bright part about OBs. Because there's no box, there's nothing restricting driver motion. The driver should realistically be well damped so bottom end efficiency is improved. However that too may mean a need for EQ. Most driver have free air q anywhere from .3 to .45 - that's just too low and means the driver rolls off way too early. You probably want a q closer to .55 to .65 so there's another need for equalization, unless you're using a high Q driver of course.