High order crossovers

since 1977.......

but to be fair, probably why RV puts powered subs in his high end gear and also sells powered stand alone subs with a unique high power high pass system and filetrs for the main amp.
your results may vary.....
I stopped mucking about with kits and an FFT with a crossover cookbook in 1986.....I outsource that to a guy with ten patents, an autoclave, latest FFt and a chamber..... 

There is one company that I think deserves mention here:

Induction Dynamics.

I've only heard their two way speakers.  They use a 4th order "brick wall" crossover in their speakers.  My (admittedly limited) experience with other higher order crossover speakers had been that, to me, they didn't have the same expansive soundstaging as did well-executed first order designs.

The Induction Dynamics speakers (remember--I only heard the 2-ways) were a striking exception to this trend.  I was shocked at how expansive and 3D their soundstage was.  Tonality was excellent also.  As I recall, the drivers were said to be "inductively coupled" or something like that.  In any case, this inductive coupling seemed to compensate for any "problems" (timing artifacts?) that may have introduced by using such a high (4th) order crossover.  

JSE (John Sollecitto Engineering) speakers from the mid -late 80's had the best implementation of high-order x-overs!  I have a pair in my collection. Later adopted by Joseph Audio (Pulsars).

No problem with the dynamic performance and coherence of my JSE Infinite Slope speakers!