MyPal, co-locating the subs (stacking) will give you better output, but will not smooth out the frequency response like two separately subs would do. The issue is the effect of standing waves upon perceived bass.
It is counter-intuitive, but the
greater the amount of bass energy in a small room the
less bass is perceived in certain spots in the room. Other spots may be dominated by a one note "boom" as the standing waves accumulate in certain spots. Adding more bass energy from the same location just makes it worse.
The reason that separate locations work to smooth out the response (yielding the perception of less boom and paradoxically stronger bass) is because each location creates a different pattern of standing waves. That combined with double the surface area of drivers allows less energy to create the same amount of perceived bass. because the standing wave patterns of the two locations are different, there is less build-up and cancellation, so less boom and "suck out."
In short, extra drivers may go to physical extension, but differing locations goes to perceived extension.
