Subwoofer Speakers add huge low end to your sound and can be added to most PA systems easily. For the ultimate in ease, go for a powered subwoofer (also known as "active subwoofers") and simply add it to an auxiliary output of your current mixer.
Some Powered subs have a through input and crossover meaning that you simply plug it in to your mixers passive output and sub will send all the high frequencies out to you powered top speakers.
You also don't have to use a pair..... unless you want to of course! Because bass frequencies are not as directional as higher frequencies, you can locate your sub over to one side of the stage and continue using two top speakers without any issue!