Agreed more than 4 derives little benefit for symmetrical rooms. Mine needs 6 before it starts getting too silly - 2 front, two side and 2 back. After that, I need an AI to system-tune the bass. It's no slouch with just two 500s up front but there's no mistaking the depth and fill the 800 provides.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated