Size and Practicality
It comes down to a balance of practicalities. The VS21 is bigger than a direct radiator 21-inch would be. Any 18-inch cabinet with a hybrid horn would be bigger than its direct-radiating counterpart.
The 21" Cone Inertia Problem
The larger mass, and therefore greater inertia, of a 21-inch cone introduces limitations on the acceleration of that cone. The result is experienced as a lack of impact or punch. Other descriptions are that 21-inch woofers are "slow" or "muddy" compared to 18-inch cones.
How the Hybrid Horn Solves It for 21"
The hybrid horn produces higher sensitivity in the upper end of the subwoofer operating range. That higher sensitivity correlates with a reduced demand for excursion in that range. With reduced excursion comes reduced demand for acceleration — overcoming the generally perceived limitation of larger cones being "slow."
Why 18" Cones Don't Need the Same Help
While some heavier, large voice-coil 18-inch cones suffer from this limitation, it is to a lesser extent. Most 18-inch cones can be accelerated fast enough to not need this secondary advantage.
The Bottom Line
Building such a cabinet for an 18-inch woofer doesn't yield sufficient improvement to warrant the added size, weight, complexity, and cost.
We ask ourselves: would a single 18-inch cabinet that's significantly larger and more expensive than the SSP118 but not as loud as the VS21 tend to serve anyone better than either the SSP118 or the VS21? The answer is no.
