First off, it is not true that "subs are nondirectional." It is true that low bass is "nondirectional," which is to say that we cannot tell where it's coming from. But it's quite likely that a sub will be producing energy up into the range where we can begin to pick up locational information. (And don't forget that wherever you set your crossover, your sub will be producing sound above that point.)
This doesn't mean that a single sub can't work. It can. Remember, if you didn't have the sub, you wouldn't hear the low bass at all. Which is worse: Not being able to place the bass? Or missing it entirely?
This doesn't mean that a single sub can't work. It can. Remember, if you didn't have the sub, you wouldn't hear the low bass at all. Which is worse: Not being able to place the bass? Or missing it entirely?