Most bass is limited by cone area, driver efficiency and input power.
The biggest problem with IB, as you've probably noticed, is that they lack mid-bass... this is because IB and sealed pretty much play full-bandwidth, the power is divided across a larger range of frequencies, and hit their xmax quicker at lower powers. That's the price you pay for wanting flat to 1hz! (In more ways than one, 1hz ain't cheap!)
ULF is almost entirely cone-area/displacement limited.
Mid-bass is almost entirely power-limited.
If you have low-power into low-efficiency drivers spread too-thinly, you can see how this is a problem.
IMO every system needs both ULF and MBM subs. The driver attributes that lend to good ULF don't make for good MBM subs and vice-versa.
One of the many reasons I chose to buy 16 PA-460's was that not only is it 16 x 18 worth of cone-area but also 8kW RMS/16kW burst at 99db/w/m x 16. Not only that, it only cost me like $1400 (ignoring amplification). An FP20k is perfect for driving 16 of those at quad 2-ohms.
You need lots of coil-area to keep things cool too. Pushing 8kW into a pair of 21's is possible... if you want them to reach the same temperature as the surface of the sun! (I smell coil!/Burnt slinky!
153db @ 1m @ 8kW
I put them in undersized opposed sealed boxes, you can barely even see them, they hide well.
If I didn't point it out you probably won't even have noticed them behind my fronts.
It's more mid-bass than I could ever use. It can kill small animals from a distance. It is so overkill that it just sounds "dumb" if you actually
crank them to their full potential, the SEOS's can't keep up and neither can the ULF subs. Even with 100kW going to them. All while producing no audible cone noise. Bass for days without overheating or bottoming. A basshead's gotta do what a basshead's gotta do! Problem SOLVED!!!