SVS, JL and the RELs S812 (I think) it's not ported are pretty serious subs..
I still believe more is better and medium excursion is better if you have enough drivers.. It's about moving air. HOW you do it, makes a big difference..
In the 80-90s I ran 4 Tall Boys. They were a 12", 15" and 15" passive in a 6cf Tall box. 1000 watt Daytons is what Brian recommended at the time. The passives were tuned to your room, amps and speakers with pinching putty from the passive to adjust (Q) uality. Thumb nail at a time.. Very accurate way to tune and blend a bass system.
OB Servo subs, both sides of the driver are loading and onloading the room, your ears act different.. I use 6 12" OB high excursion drivers... they work both ways not one.. Decoupling those cabinets made a heck of a difference. They are made to stack too. I ordered 2 more double 12", we'll see.
Regards