Joe X

You will need like 12 square inches of port area for your box, anyways since you are in a SUV now you may not even hear that single 8" at all sealed or ported, given the vehicle size I would be thinking of a single 1K 15" sub sealed at least to accomplish what you want.

If you want output but you don't want to be peaky you go with a higher diameter sub and do sealed, box would still be 1 cubic feet or less, just a little taller, you can use your own 12W7 in a 1.375 cubic feet net box or pick a 12" that works in a smaller box. The SA8 will still be peaky in a 0.6 net box and will give very little output in sealed.

Two component sets will need 4 amp channels, not sure what you mean you are running 4 sets.

There are many options but yes, you could pick some Xs for the low end or Us for SPL type of setup whichever suits your taste.

4th order bandpass drops output above tuning leaving you with no output usually 55-90Hz leaving you in need for midbass drivers to fill in the range, unless you are ready to install midbass drivers ported is a much simpler alternative, 4th BP are good for blow through enclosures where you need all your output through a small hole. Two higher power 12s ported will usually be the best solution in your type of vehicle.

Uneven loading is what can happen if the sub in question is too close to the internal end of the port.

When you have 3 subs you do something like this:

There are options if you already own the subs including actually mail them, if not by all means pick something else, most of those no name companies just import the subs from china, no actual manufacturing taking place in Panama whatsoever.

You said sub and now you say subs. How many?

Very small and efficient, being used by at least one member on here without complaints: https://www.bladeice.com/product/b2-audio-riot1200/

Contact them: I bet they don't know either but it's worth a try.

What are the maximum dimensions for a box?

The SA-10 in a 1 cubic feet ported @35Hz is peaky and loud (orange plot), 0.5 cubic feet sealed (yellow plot) is flat (but silent): Using heavy damping you can make it less peaky as you can see: The box itself can be like this:

I say forget what the ideal size and focus on what is best for your application, if you need a small enclosure the best approach is going with a sub that works well on smaller sealed enclosures. You can use advanced modeling software to help you determine more exactly the effects of the damping material as well as the enclosure itself.

The flattest possible response inside a car comes from a sealed enclosure, 0.6 cubic feet net per driver or so should work fine. You don't need to build them, high quality prefabs of the correct volume can do the job.