When I added three more subwoofers it absolutly negated my rooms nulls and modes. Since the subwoofers have extensive multi band multi parameter processing the ability to better match the main speakers full range presentation in the crossover region was not subtle.
I've since removed two of the subs which resulted in a mode in an unused corner of the room. With every rooms uniqueness this may not be as desirable.
Mike's is good advise. The ability to see how your room is handling the low frequency is very useful. Along with repositioning and some form of processing you may find a level of satisfaction before committing to more subs. Good luck with it.