NIck, ideally I think you should get matching subs, but sometimes that's just not in the cards. I run multiple subs from different manufacturers and have good results. The main sub is a Hsu VTF3 mkII followed by two smaller 10" sealed subs. Yeah, it looks horrible on paper, but the smaller subs do a good job of filling in the gaps created by room nodes. I've always run them in rectangular rooms.
On the other hand, grunt (Dean) has tried out multiple subs (I loaned him mine to test) in his listening room and had worse results than just running his single EP500. His room has some odd nooks and crannies and I wonder if those do weird things to the room response?
For the most part, I think running multiple subs is beneficial.
