Many people have a high expectation for the performance of
a single in-wall (or invisible) subwoofer.
Let’s settle this now: one built-in subwoofer will not replace the performance, output, extension, and impact of one high-end cabinet-based subwoofer.
The LWSub can reach low (to 25 Hz -6dB), but plays 25% as loud as a high-end 10" powered cabinet-based subwoofer.
Therefore, you will need at least four LWSubs to perform as loud and low as a single high-quality box sub.
Use several invisible subs per room for high output
We have clients who demand theater level sound from their invisible speaker systems. No problem. Simple math tells us how many speakers and subs to place in a room to achieve high level output. In general, a 6-seat home theater requires 6 or more LWSubs for best performance. (See: How many speakers do I need in a room?)
The benefit of multiple subs
Although you will need two LWSubs to equal the sound pressure level of a single box subwoofer, you achieve the serendipitous benefit of multiple sources of bass: improved coverage throughout the room.
A single subwoofer in a room is problematic, because it excites room modes in a given manner. Most rooms will create location-specific bumps and dips in the frequency response. And based on typical seating arrangements relative to the walls and speakers, the most egregious bass fluctuations tend to cluster around the listening area.
Adding a second sub — and strategically placing it for best impact — smooths out those room modes. Adding a third and a fourth sub — also strategically placed — adds more smoothness. So while it takes two LWSubs to achieve the same output as a single powered subwoofer, it's most likely the room will sound significantly better with two LWSubs versus the single powered subwoofer.