Much good advice above. A 15 won't necessarily produce more lows than multiple smaller drivers. Chris B raises the important point of efficiency/sensitivity - how much acoustic output you get for a given input power. That is down to cab design, as well as driver type/size. Simply adding a large single driver cab won't necessarily give you more of what you want, especially if it produces a lot of subsonic or infra bass, which can cause issues with the PA via low frequencies travelling through the stage floor, up mic stands and into mics. It can also muddy the sound in the room.