@shadyJ the response curve on the Ultimax, while pretty good looking on it's own, doesn't integrate that well with the others. It made sense to me that their relative strengths and weaknesses would be complimentary, I've been having a pretty difficult time trying to flatten things out.
@Pogre My issue is getting a response without a couple of big suckouts- output isn't anywhere near a problem, haha! I've created filters in REW that address those suckouts, but addressing one frequency has a collateral effect on another. It becomes this Cascade effect, if you will.
My general thought is to have 4 subs that, while not completely identical, are tuned similarly to a point of being able to eq them as flatly as possible, then be able to see about creating a house curve to tune the response to my liking.