I would expect it would be preferable to have two or more subs of the same manufacturer but I've read many accounts of people successfully integrating different subs in their systems(at least of similar output/quality).

I'll also throw this out here for discussion....I was talking to a local P'digm dealer who insisted that a good SINGLE sub works better than multiple subs using the two drops in a pond analogy, that soundwaves may have the effect of cancelling each other out when they collide.
Half of communication is listening. You can't listen with your mouth.