is there really a need for two subs?

I pretty much agree with Sdcampbell.

But if I were on a budget (for which I am) I would take one good musical sub over two mediocre subs everytime.

The quality of the sub means everything. And this is one region where it's better to do without than to do with poorly. But it can be done.

And don't settle for a sub that stops reproducing at 30Hz or even 25Hz. Any decent full range speaker can pretty much handle that region. Ensure that the sub will reproduce faithfully to below 20Hz. That's what a sub is really for.

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?

I have been using two subs since the year 1981. Stereo subs greatly increase soundstage size, shape, and realism.

I agree with some of the comments above. There is a level you have to reach first before two is better than. Generally, one $1500 sub will out perform two $750 subs. Two really good subs will be even better. The sub does not have to do double duty with the bass signal from 2 channels.

you need to decide if you want nondirectional bass or quality non directional bass as it affects your music enjoyment profoundly for all the reasons stated above by other members.