Well first of all thank you for your patience.
I have had a lot on my plate lately. This speaker design ended up being a difficult and complex problem to make an elegant design for those drivers. However I'm now happy with the result.
The first file is the proper design for your JL Audio 12" woofer. I have no idea where JL audio come up with their box design it fails on every count. And so do the ones for the 8" driver.
Now I have done designs for two separate boxes for the JL 8" drivers and two to a box. I would recommend you make two singles. That way you will have a speaker with three divided enclosures which will add to strength and bracing. So you would have one 8 driver above and below the MTM enclosure. I think this will be the easiest build also.
I have done a design for the JBL MTM sealed enclosure.
Lastly there is the file of the Crossover for the JBL mids and SEAS tweeter. I had to change tweeters and had to search for a suitable one.
It is this SEAS tweeter.
Now there is one component I could not place in the circuit. You will see there is a rise above 6 KHz in the model. You need to place a 2 mfd cap in parallel with the resistor Rp2. If it sounds a little bright you might have to go as high as 2.5 mfd. I had to do that calculation manually.
Now the bass enclosures will need light fill with about 50% of the surface covered. Use Polyfill. The sealed MTM enclosure will need completely filling with polyfill. Do not compress it.
Now you will need to add the following to all box volumes. Add 10% to cover driver displacement. Add the volume of all bracing and the slot vents.
Now the crossover from the 8" woofers to the mids need to be active. 200 Hz will be the ideal crossover point.
I suggest this Behringer unit. Sweetwater are excellent to deal with, and I would bag that demo unit.
I would engage the 25 Hz high pass filter on that unit. It will allow you to balance out the LF and Mid section.
Now for amps you will need a couple of 150 to 400 watt dual channel amps. They need to be four ohm stable.
Now some notes on the crossover. Those mids are not as sensitive as specked when you deal with the 3.5 KHz breakup mode. You loose about 9 db of sensitivity.
The crossover does have a short 2 ohm null at crossover, but this will not upset any decent amp.
The phase response, impedance response and group delay are spectacularly good.
I would not cross the main speakers over to the sub. Use the 25 Hz high pass in the Behringer. Gently bring your 12" sub starting at 60 Hz.
This should be a very good and potent speaker and be well into the 10 to 20 K per pair speaker range.
This design was a lot of work so I hope you build it and post lots of photographs.
If you want me to build the passive crossovers for you I can, and we can talk about charges.
If there is anything you don't understand, ask.