This thread reminds me of my car audio days back in the 90's...right before my internet addiction started and I had time.
A buddy of mine, he was 21 years old and wanted the highest SPL yada, yada, yada and initialy, a 7 cubic foot box was not a problem. My hormones were under control by then and I realized he would soon tire of something that huge but I'm not the audio nanny. I spec'd a pair of beefy 12" subs and we built a 250 pound box that took up his hatch and back seat--because more is better.
The pair of 12's were wired push-pull opposed which gave a few benefits. For him, he wanted to see the fancy sub basket he paid for so it was cone out on one side, magnet side out on the other which garnered cool points (well, it was cool if you were a guy but the girls thought it was ugly) Since audio is about benefits both real and imagined, the push-pull reduced distortion, allowed the sealed subs to run cooler (magnet side out cools better) and prevented his car from vibrating.
Eventually, it took about a month he tired of the magnet side out and the worry about cables, damage etc. and went to conventional opposed. Eventually he obtained a thing called a "girlfriend" and he meekly asked me if we can make the thing smaller. Sure! I cut the box in half to 3.5 cubic feet, slapped both subs in opposed and he was happy.... "for a time" He then wanted the sub to be even smaller, so I told him he could either have a single 12 in a 1.75 cubic foot sealed box or load both of them isobaric in the same box but one of them would be magnet side out and it would lose efficiency--but he can drive both subs with more power AND it would not vibrate...reduce distortion etc.
He ended up with isobaric clamshell magnet side out in a 1.75 cubic foot sealed box. Ya see, when testosterone is involved you need a "reason" to be a wuss....errr, call it "sound quality" Yes! That's it!
The Iso sub worked well, did not vibrate (although SPL will shake your car to piece anyway) had poor efficiency but could take the power. Yup, part of the car audio game is WATTS and the Iso could take the watts...for the win! He also gained cool points for magnet side out to show those fancy baskets. He eventually broke up with his girlfriend, she returned his sack (she kept it in her purse) but his thrill of driving a portable subwoofer around was over. Such are the trials and tribulations when you date strippers.... they don't mix well with car audio.
My garage subs are pair dual 15" 8 ohm drivers configured at push-pull slot loaded subwoofers, they generally run ported but can be sealed by design. They do not vibrate, I can impress audiophiles and idiots with my supreme building skills by balancing coins on them! (Ooooh, ahhhh!) They have line arrays with over 80 drivers each placed on them and avoiding vibration with over 200 solder points per speaker is a good idea. Since my woofers are enclosed in a slot, it prevent drunken idiots from damaging them, crap flying around in an active garage hitting them and so on. That slot puts the drivers under pressure as they have a 2.2 to 1 compression ratio which....uhhh...efficiency increase etc. The kids like it because it increases air flow out of the slot which is always cool. It also naturally reduces distortion and the big one--looks unique to people or they tell me I put in one of the subs backwards. The biggest downsides of that configuration is you need to use quiet subs because the magnet side out, if it creates a lot of air turbulance you will hear it--not a good thing. The slot takes up space so the box will be bigger and it must be built correctly to maintain a seal. You also limit the bandwidth of the sub, not an issue if it is a true sub but if you want to go higher than 200Hz--better do the math to find the limts when loading subwoofers/woofers that way. Once you break 300Hz or so, slot loading creates issues so do the math.
Overall, if you use two subs per box--attempt to at least go opposed to gain something. Push-pull is better, it gives additional distortion cancelling but you need quiet drivers, driver protection etc. (not good for children or strippers) I find push-pull slot load to be the best because the drivers are safe, you can put a grill over the slot to hide them and no vibration is a good thing--if you just want subs, not worried about build complexity or size. Isobaric also works but very poor efficiency--unequal cooling of the drivers depending on configuration. However, it is very small and sounds great if you do it right.
Although dual balanced drivers gain cool points--the ultimate in cool is to have a push-pull horn loaded sub--either front loaded or tapped horn--your choice! Since horns by their very nature are heavy, complex, giant beasts--most of them use push-pull when loading them with dual/quad/octal drivers to improve reliability and performance.