Todd Welti is one of the foremost experts on subwoofers, and his top choice is four subs placed at the midpoint of all four walls. But he also said two subs at the midpoint of either the front/back or side walls is still very good. I’d start with that for placement and use something like a miniDSP/REW to do the fine tuning. I’d recommend reading Welti’s work to see his research that led to his conclusions. I think this video hits on a lot of the key points if you’re more of a visual learner. Hope this helps, and best of luck.
