I consider the subwoofer to be an integral part of the speaker system, just like the woofer, midrange, and tweeter. The fact that it happens to be in a different enclosure is a detail. Accordingly, I think that each main speaker deserves its own subwoofer, colocated with the main speaker, and driven by the LF signal component of the particular channel (not a mono SW mix). Thus, for my multichannel rig I have three SW (for the fronts) and will some day get around to installing SW for the rears. In my view the main function of the SW is to avoid extreme excursions of the main speaker woofer cone (or planar diaphram) so that the main speaker can do a better job on the upper bass.
Why dual subwoofers ?
Just got a new processor ( EAD 8000 pro ) that has outputs for two ( stereo ? ) subwoofers. What advantage is there to going with two subs ? Does it really make a difference ?