Hi Andrew,
Band pass boxes, in my opinion, are as hard if not harder to build CORRECTLY than the Lab Sub. The 6th order you refer to has 2 resonant chambers, one tuned to one freq., one to another, each to theoretically cover a wider frequency band. Hard to mate a "off-the-shelf" driver to that with those peculiar parameters AND high power handling.
Most of the BP boxes I've heard sound terrible (with the exception of a NEXO S.2) and don't go very low in the small box designs. They seem to "latch" onto one note in the program material and moan away at that one frequency. The more of them you stack together, the worse it becomes. Maybe I just haven't heard a good design implemented yet. I've only mixed 2 1/2 decades now.
I found it difficult in the extreme to mix on Manifold boxes in the 80's and 90's, and that was, I think, the pinnacle of BP box development. The drivers were always blowing and sound companies were always showing up with 2 or 3 drivers working in a box.
I ALWAYS went to the crossover and created a "hole" at 150 to almost 200hz if I could. The quantity of bass was great and it reproduced the kick signal with abandon. The bass guitar response SUCKED. Wolf notes everywhere. It was just a complete nightmare if the room co-resonated (in clubs usually the long wall distance in feet x 2, a la 200 hz) and they usually DID co-resonate a LOT. I could almost NEVER get smooth freq. response in the majority of the venue. There was always this "sweet spot" and most Sys. Engineers made sure mix position was there! It always formed a triangle with equidistant sides with one side being the center distance between the 2 stacks. Outside that triangle there were all kinds of cancels, nulls, buildup nodes, you name it.
Try to mix on that every day!
The reason the NEXO works so well is it only performs in one octave (32 to 64) and it is electronically controlled, actually making it a 7th order BP. It distorts, no doubt about it, you just can't HEAR the distortion very well in that octave. It arrays and throws well, packs good on the truck, and only weighs 187#, so it is a "good" BP box IMO. Only thing that takes getting used to is it's a 3 ohm box. It takes a big mondo amp to drive it.
Is there a design you've found and prototyped that works to your satisfaction?
Dan 0;)