The comparison you mention, with the 400 on top of an 800, is kind of specious, at least in a large room. The 400 was designed for deep bass extension in a small room, so it isn't capable of the same SPL as the larger EP800. If tactile impact were the criterion, the EP800 would win, simply because of its greater output capability.
I am trying to really understand what contributes to impact, which I why I am comparing two very different sealed subwoofers. Both are sealed, but the EP800 can obviously play lower than the EP400 and at MUCH greater SPL levels. But, I was trying to take the extra extension and capabilities for much higher SPL's out of the comparison, by say comparing them in a quite small room and having some external EQ that cuts out the extra extension of the EP800. So, under these circumstances what would be the difference between the two subs in a bass heavy movie scene? Would we be able to feel the extra impact using the EP800 in a bass heavy action movie, in explosions, gunshots, trains passing by, etc.? As long as we aren't pushing the EP400 further than it can go. Or, is the reason for going moving up the Axiom line (or other subwoofer manufacters, as well) just a matter of extension and high SPL capabilities, needed as rooms get larger.