Outdoors, the problem is you have a lot of area to energize. There is no room gain. But you don't have standing wave modes within the walls to worry about either. Just the nodes that might setup between subs, if placed over 1/4λ apart.

Inside a room, the problem is room modes. These are determined by room size and proportion, subwoofer number and position and listener position. Large rooms are typically less problematic where room modes are concerned, because modes shift down in frequency. The larger the room, the more it acts like open space. Smaller rooms generally have more noticeable room mode problems. One solution involves using multiple subs strategically placed to partially cancel peaks and fill in the holes caused by standing wave nodes within the room.

Welti suggests four corner placement or four subs placed at wall midpoints. Geddes prefers random placement. I tend towards a staggered symmetrical approach, one placing subs in different places in all three planes but symmetrical with respect to the listener. Each placement method has its strengths and weaknesses, and many of them are room specific.

What works best for you?