The laws of physics prevent co-locating two subs in exactly the same place. Placing one on top of another can actually help smooth out bass response. There are floor to ceiling standing waves, having a sub placed higher or lower can allow it to couple with different modes even if the left to right and front to back don't change.
As a matter of fact, if trying to avoid boundary gain, I don't understand why so many subs place their cones within inches of the floor. Most of Axiom's get to cone up pretty high (although the ports may be a little low); the vertical EP600 and 800 completely avoid that problem (as long as you don't stuff them in a corner).
I'm probably going to get an
ASC SubTrap, and see what it does. Although I may try a stack of 4
Auralex SubDudes, as I've read about people having good results with that for much less money (although it only eliminates boundary gain; performs no actual trapping).
