jOnhwa11 I can tell you're new at this so I'll try to explain it. Just because a pair of mains are large doesn't mean that they are the best for producing bass. If the mains are truly full range speakers. Most aren't. They will suck a lot of power from the receiver or amplifier trying to produce bass by themselves. By adding a subwoofer there are many benefits. The subwoofer has it's own amplifier. The subwoofer can be placed in a proper location in the room for best bass response. You can't do this with the mains. By using a subwoofer with large mains the sound will also be more dynamic. For instance my mains can play down to 25hz. They do a pretty good job on their own with bass, but I have ample wattage available. The room sounds even better with the mains crossed over at about 50hz with multiple subwoofers doing the rest of the work.