Powered subs - now there is a loaded topic too. Mackie makes a terrific one that has a stereo crossover built in, so only one is needed - and you can still maintain stereo imaging. The JBL counterpart has an 18" speaker and really kicks serious bass butt.
They are both in the thousand dollar range.
If you economize on the bass bin - you will work your tops harder, and that may or not become a problem in the long run. Samson makes a mid priced 500watt sub thats probably as low as I would suggest for you. Try to have at least twice the power for the bass as you do for the tops. A better equation is really about 4 to 1 ! Bass is very stingy with head room, and you can NEVER have enough. To make bass, you have to MOVE air. Period. Watts don't matter - size don't matter. MOVEMENT matters. Listen to all the choices WITHOUT the tops connected and see which one makes your tummy rumble. Try a crossover point about 120 - 250 (max). I personally like 180 with my EV's as tops.
The human ear cannot distinguish directionality in that low frequancy range, so placement is mot important either.
If you are not really thumping some MAJOR bass out - the samson should be just fine to round out the system. If you are doing large venues all the time and the bass is kickin alot - go with a larger system. It's always wise to have a little "extra" for when you need it.