Single Rel Storm 3 works great in my system. Downward firing, with crossover set at 27 hertz. No longer made, but readily available used. I've been mostly UNABLE to localize it, even though it's in the corner. Deepest bass usually seems to emanate from the middle of the two speakers, even though that cannot be happening. A wonderful and sometimes hilarious thread regarding the merits of one versus two subwoofers has already been posted. Aside from the entertainment value of the arguments, what I took away from it is that stereo imaging becomes more of an issue above 60 hertz, and that it's less important below that frequency, because the deepest bass is nondirectional.
One versus two subwoofers
I don't know the answer, but can verify that in my system a single sub-bass unit suffices insofar as imaging, with the deepest bass seeming to come from exactly between the monitors, or sometimes JUST off center towards the side with the sub, but still between the speakers. I'm talking about a perceived shift of only one to two feet. I don't claim to fully understand it and am open to the possibility that my auditory perceptions are flawed.
One versus two subwoofers
I don't know the answer, but can verify that in my system a single sub-bass unit suffices insofar as imaging, with the deepest bass seeming to come from exactly between the monitors, or sometimes JUST off center towards the side with the sub, but still between the speakers. I'm talking about a perceived shift of only one to two feet. I don't claim to fully understand it and am open to the possibility that my auditory perceptions are flawed.