"LTD02, I was hoping you would chime in! So in the scenario you have there, I have the space, but which ones should be the sub bass SI-18's and which ones the 4645clone?
Im thinking the two flanking the center should be SI-18's?"
yeah...si's in gray in that pic. no particular design, just roughly idea for placement. since they will cover the bass region where the room modes are sticking them in the 1/4 distance positions is where they will have the most benefit.