Deciding on the number of powered subwoofers for your home theater setup largely depends on the size of your listening room and your desired audio experience. For an average-sized room, about 1500 cubic feet (roughly 14' x 14' with an 8' ceiling), a good quality 10" or 12" subwoofer usually produces sufficient bass levels for most listeners. In this typical setting, one subwoofer is often enough to provide a satisfactory audio experience.
However, if your room is exceptionally large or if you desire more extreme output levels, using multiple subwoofers can be beneficial. In larger spaces, a single subwoofer might not evenly distribute bass throughout the room, leading to uneven sound quality. In such cases, adding a second subwoofer can help to "even out" the bass response across the entire room. This is particularly useful if you find that the bass sounds good in some parts of the room but is lacking in others.
Ultimately, while many standard home theaters are well-served by a single subwoofer, adding a second one can enhance the overall sound in larger rooms or for those seeking higher intensity in their audio experience. Remember, the key is balancing your room’s acoustics with your personal preferences to achieve the best possible sound quality.