One of these times, if you're inclined to experiment, grab a copy of CARA and model your room. Place woofers in various places in the room to find what makes the most uniform response. Simulation takes just a few seconds, and it will show the modes in the room as a function of frequency. It even makes an animation to demonstrate the modes with a simulated frequency sweep.

I would start by placing the two additional woofers in different positions in all three planes. One configuration that seemed to generally work well for me was to separate woofer pairs by a few feet, one higher off the ground than the other. Try other configurations too.