High
acoustic impedance is what big drivers have that makes them better suited to playing LF than small area drivers.
Bigger drivers grip the air better, and that provides a lot of advantages. Air slippage of a small cone is distortion, lost efficiency and lost detail. Bass drivers will never have the kind of acoustic loading that tweeters do, in terms of a ratio of diaphragm area to wavelength created, so we will never get that level of detail from bass drivers.
The advantage of a big driver over a small bass driver is similar to rowing with oar facing the water load, compared to cutting through it with the oar edge. The flat oar moves the water particles, creates a LF pressure wave whose opposite reaction moves the boat. An edge-slicing oar moving at the same speed simply allows the water molecules to slip by. But if you move the oar edge quickly enough through the water, the impedance at the edge would increase enough to move the boat. Same as a tweeter only needs a small dome, because it is moving faster.
A small woofer moves too slow to launch a wave efficiently. The air just spills out of the cone rather than explode forward. A big driver spills some air over the side, but some still remains in the center unable to escape and this trapped air causes the wave launch. The bigger the driver, the more air is trapped on the surface, cruched against the cone to create pressure, then released as the driver reverses to launch the wave.
When the small driver plays louder it is moving at faster velocity even though it is the same low freq. This is why small speakers need more volume to wake up and sound good. This is also why big speakers sound so satisfying at very low volume level, they grip the air and make clear undistorted bass even when they are moving very slowly on low notes played very softly.
The only potential problem with large drivers is overpowering a room. A sealed driver falls off at 12dB/oct, which matches the rising response of the room supporting FR into low freqs. When the sealed driver is sized correctly, it's rolloff will match the rising response of the room to make flattish FR. If the woofer is too big, the perceived FR will increase into lower Freq. This is very easy to correct with a single cap to make a high pass filter to cancel the net gain of the driver over the room. The shittiest cap in the world would sound fine at 40hz. The added dynamic range and tonal detail that you would gain from the oversized cone would more than make up for the hassle of adding a cap. This is why people put twenty 15" IB subs into their walls. It's all about detail and dynamics. The level is still matched, it's not about the loudness. But they want their subs to sound as detailed and exciting as midrange horns. Bass horns are a good example of this. A horn is nothing more than an impedance transformer.
edit: Actually, unless the woofers are directly next to each other, they can't combine forces to lower the impedance. If there is space between them, like a swarm of subs around the room, there is no impedance advantage over a single sub, they won't pay lower, only louder (and better room modes.) But if multiples are adjacent like a line array then impedance can increase and the array can play lower than individual drivers alone.