well I am by no means an expert by any strech, but I would recommend that when deciding on how many subs to use with any given system you must take into account all the variables (room acoustics/size, system pairing with, etc...) and what your are going to use the system for (theater room, music type, personal tastes, etc...). For most setups (small to mid size) two subs are plenty, but if your going large, 4 can work.