...but keep in mind that the subs start becoming more directional or, more precisely, you may start getting directional information in the upper range of the subwoofers - hence my recommendation to use only one sub (if possible) and place it on axis in between l/r speakers, with the sub firing sidewards and the speaker as close as possible to the front wall (different story when flush mounted obviously):
this leaves more options for x-over adjustments and coupling of lf sources, having a huge effect on the overall speaker system response and how it's interacting with room geometry (someting somewhat neglected in many places imo), but can cause other problems (and i'm sure jens will remind us with the related link).