The eight would bottom out before getting to an acceptable volume level.
It has to do with the subwoofer design.
To get a low note, you need to produce vibrations in the air that are spaced widely apart. To get the volume, you need to displace air.
With a larger cone area, you need less xmax to produce a low note than smaller cone area, which needs a lot more extension to produce the volume, and as such, it would bottom out trying to keep up.
Manufacturer boxes are the ones designed to give the subwoofer enough airspace to both not choke the sub, and to give it enough resistance that it won't easily bottom out and you can run the rated rms power to it.