Originally Posted by SteveGTR ➡️

Yes, the low frequency energy is summed to mono and sent to all 4 subwoofers

There are multiple approaches to 2 subwoofers, but in the one that is connected to the Harmon/JBL research which recommends 4 as ideal ,you would also sum them with the same signal. In this case the best results are often achieved by having one at the midpoint of the front wall and one at the midpoint of the back wall.

Other approaches include having them under the mains facing each other or treating them as separate signals so that L and R mains and subs are full range, discrete signals