Most main speakers are typically passive. But most subwoofers include a power amplifier, creating the raw power needed to provide deep bass, without taking away any excessive amplifier power from the receiver. Subwoofers usually handle frequencies below 150 Hz or so.
Most subwoofer are designed to deliver accurate musical bass from a small enclosure. Small enough to be placed behind furniture or tuck into a corner and not be obvious from view. But its big bass output will be able to deliver all the visceral gut rumble for the action movies as well as tight, accurate bass precision for music.
The trend toward small, easily concealed main speakers has fueled an increase in the use of subwoofers. Compact main speakers, and in-wall or in-ceiling speakers, are often bass-shy. Large, costly floor-standing speakers often have plenty of bass, but they may be cosmetically unacceptable. Also, the speaker room placement that results in the best stereo imaging is almost never the best placement area for reproducing deep bass.
So don’t make the front stage speakers do all the work. Instead, add a subwoofer to the system. This will free up the front stage to be more dynamic and expansive by letting the subwoofer take over the low frequency effects, the thrills of movies and the deep bass of soundtracks. The design of the intelligent amplifiers in subwoofers basically filters the low frequencies cleanly and smoothly, for better blending with the full audio spectrum being reproduced the main speakers. Easily accessible, professional-grade connections will give multiple options for hookup, so that one can tweak the performance according to the system needs. Adding a subwoofer, or two. will make TV watching into a wild ride, and music listening into a real live-on-stage experience.