Multiple subs can deal with room modes, where one sub can not. Because LF has such large wave lengths, there are very few rooms that can properly integrate one sub. The rule of thumb is two subs fix most of the issues, a third can help and a fourth is best. You get less and less out of each sub, so four subs are better than two, but not twice as good.
I ran one undersized sub for almost a decade before going to a 2nd even SMALLER sub. It didn't make much difference. Recently I bought two Rythmik F15HP (same one's as
@FrantzM suggested) and WOW! I use my original sub behind the seats for a nice seat rumble effect. Two quality subs, capable of handling your room, are highly recommended. If your budget allows and your room can hold them, four would be better.