The rolloff in the EP400 is not typical with what you see in a sealed subwoofer especially in a box and driver of that size. Its a small box and a 8" driver.
The reason why its not typical is because the EP400 uses DSP processing and it is trading off output for extension. The EP400 extends really deep for a subwoofer of its size but if you notice it is still a sub that is recommended for a small/medium room or den and not meant to replace the EP500 in a large room for example.
I'm not really sure the exact reason why the brick wall filter is employed but it is probably to keep the sub operating within its proper bandwidth as its specifically tuned for extension. I could be totally wrong though.
