advice needed on spl system

I am putting a wall in my 97 ford ranger extended cab. I have 18 cubes to play with. I am planing on either 6 15 s on a flat wall with 3 cubes per sub while running two amps on them. I see everyone using the 4th orders though, so my question is would it be better to use 3 15s in a 4th order bandpass to try to get the most bass/ spl out of it? so i guess here are my questions.....

6 on a flat wall or 3 in a 4th order?

should the 6 be facing forward or would it be better to have the subs facing each other with three subs facing up and three facing down dividing three subs with a divider in the middle...

any advice would be greatly appreciated.... thanks

you planning on cutting a blow-through and using the bed as extra airapace? otherwise i cant see how you figure you have 18 cubic feet in the back of that ranger. i guess you can get it if you build the box inside the truck with no hope of ever taking the box out in one piece and only using a single layer of wood for your box, a big mistake for a big SPL enclosure. the back of that ranger is roughly 4ft by 4ft of usable space and depending on how far back you want the seat to go you will have maybe 15 inches of depth if you still want to drive this ranger on the road. after taking out volume for the thickness of the wood, id go at least double layered which puts you at 1.5 inches all the way around, then take more volume for bracing and then take more volume for 6 15's id guestimate your volume closer to 15 cubes sealed. if you go with a 4th order then you will have even less to work with because there is a lot more internal structure involved. i say all this because i just built a wall in a 2001 ranger for two 18,s ported. the box was double layer MDF, it had to be built in two parts so that it could fit through the door with the seats taken out. you can only build the box about 6 inches shorter than the total height of the door because you need to leave room to maneuver it into place. the second part of the box took up almost the entire bed of the truck with about 15 inches left to spare from the back of the box to the tailgate. with a box this size it was still only 16 cubic feet of airspace after displacement. im not trying to discourage you from doing this, i think it would be awesome but i have a feeling that you havent thought this all the way through.

what do you think would be louder,  a 4th order with 3 15 in subs or a flat wall with 6

oh and the box ill be built in the truck.... coming 19 inches from the back wall, not 15,  51 inches across and 39 high..... also that's not considering going all the way to the floor.  I measured the height from the top of the back hump so i'll get another cube and change there too.  and around another cube if i slant toward the side glasses...

if built correctly, the 4th order would probably hit harder on a meter but it will be much harder to design. the pros of the 4th order would be that you wouldnt need as much power because its only half the woofers. even a sealed box with that many woofers needs to be divided and sectioned off so the woofers dont interact with each other and cancel and will need a decent amount of bracing too. ive always been a believer that more is not always better so id go with the 4th order for numbers but it probably wont sound very good.

I know I late with any kind of response but I have a question. Are you planning to use this exclusively as an SPL vehicle or will you still be driving the truck on a regular basis????there can be only one.......

Anybody considered that this question and the answers submitted do not take into account what drivers will be used or what exactly everyone thinks a 4th order is (an alignment or a physical configuration, bandpass, vented, assisted)? Because various configurations have various attributes. When big power and a large quantity of drivers is not an obstacle (what appears to be the case here) then Isobaric configurations can often be of benefit when using large diameter subs. Isobaric may not be the ultimate solution here but keep in mind that it has fallen out of popularity largely due to smaller subs becoming more prevalent. If simplicity and high output is the goal the I suggest drivers with a higher mechanical Q in a sealed enclosure (single or isobaric). Not off the charts high Q, but rather higher than you will find on some of the more powerful super subs on the market. For example a JL W3 instead of a W6 series. Though JL does not make a 15 it's a good example.