The best live performances are the ones that the audience can feel as well as hear. This is the domain of stage subwoofers, which have two important jobs to do: they reinforce the low frequencies that don't come through as strongly on other speakers, and they deliver the physical part of the music that gets the venue shaking (literally). Every live sound setup worth its salt has at least one subwoofer on the stage, carrying that bottom-end thump and rumble.
Most of the subwoofers available here are unpowered, which means they don't have their own internal amplifiers. This makes for a versatile setup since you can use your choice of power amp to customize the sub's performance. On the other hand, if you're looking for something that's easier to transport and set up, you might like having one less piece of equipment to carry by choosing a powered model. The decision is up to you; it's simply a matter of preference.
One thing to keep in mind when picking out your subwoofer is the design. For example, do you want a traditional sub or a folded-horn model? The difference between these is mostly in the way they project sound. A standard driver pushes directly on the air directly in front of it, driving sound without much direction or control. That's good for most situations, but it can cause some of the bass to be lost at the further ends of a large venue. That's when the folded horn comes in. These subs are designed to throw the sound, which makes it easier to fill an entire room. Of course, if it's an outdoor concert that you're gearing up for, an even better bet would be a line array. This setup lets you chain a whole series of subwoofers together, creating the most powerful setup possible.
It's easy to think of subwoofers as the domain of bass guitars and other low-frequency instruments, but the truth is that any instrument sounds better with subs included in the live sound setup. When you want to put on an amazing show with no compromises, equipping yourself with stage subwoofers is a must.