Interesting bit there is that Ian says that a single EP400 doesn't have all that much output (of course, all that much output for Ian vs all that much output for me...), but has great output when you start using multiple subs.
I believe he meant compared to the relatively huge EP600 which has powerful output. Keep in mind that the EP600 with its 12" driver and 600W amp is over 5x as large by volume as the 400 with an 8" driver and 500w amp.
Regarding output, one way to look at it is that the EP600 is twice as loud as the EP500 which is twice as loud as the EP400. You would need to colocate a pair of EP400s to match the output of an EP600, the extra 6db coming from the coupling effect of the 400s in close proximity. Separating the subs by much more than a couple of metres loses much of the coupling effect but you get better smoothing of room nodes. That's why he is recommending the use of multiple 400s, ideally 4 of them, as the best HT sub solution which reduces the need for electronic equalization and provides all the output you would want.
