Q : Why can't you put DD subs in a sealed box?
First off, we do have subs that can be used in a sealed box. The 1000 Series have lighter suspension and work pretty well right out of the box. All DD subs will work better when they break in.
Regular suspension DD subs are designed to last a very long time, the suspensions take time to break in, then they stay consistant just about forever. A fresh DD sub will have a high resonance frequency and a higher box resonance than a sub which has a loose suspension. It will take time for the DD sub's resonance frequency to drop.
Sealed boxes are on the low end of the scale for power efficiency and much of the energy built in to a DD sub will be wasted. One might expect to get 50% or less of the potential system energy.
If a sealed box is being picked for smaller box volume, then we suggest the use of the DDbox system to pick the correct box size first, not pick a driver diameter first. We developed this system to deliver a higher percentage of system efficiency. We have high efficiency subwoofers that will work in boxes with less then 0.4 cubic feet of airspace. One of our 8" woofers in a DD box will outperform a 10" or 12" driver being used in the same size sealed box. The 8" will play deeper, louder, cleaner with better sound quality.