An additional subwoofer provides the following advantages:
1. The second membrane surface distinctly increases the sound pressure.
2. When using an additional identical subwoofer, the following also applies:
From a purely technical view, the performance is doubled, yet the level only increases metrologically by roughly 6 dB which corresponds to a doubling but which the human ear cannot truly perceive. The sound is louder but is not perceived as twice as loud.
3. In some rooms, the room acoustics can be significantly improved by a second subwoofer. Every room has a room resonance frequency that is additionally amplified by the room due to the frequency emitted by the speaker - the result is often droning or reverberant basses. A second subwoofer can accurately counteract this frequency; the result is a dryer bass.