I think you should place your subs in locations which result in the flattest, deepest response, regardless of orientation...facing subs down or drectly against a wall serves to increase room-boundary effects and loads the room more efficiently, but sometimes at the expense of even response throughout the room. Since your existing subs roll off at 30hz, corner placement could help extend the response (unless they are reflex enclosures), but that could end up exciting room modes excessively. Since you have multiple subs, careful placement can even out the response and prevent the nodes and nulls. There are plenty of threads and articles here and elsewhere on sub placement, and experimentation is the key to success.