Been following this one with great interest, since i've been using folded horn subs for PA for a while. Wayne, look at www.stage-accompany.com and look how they have employed forced air cooling into the performer series. ("Air system").

Basic form is a fan, and a speed controller that reacts to averages of progam levels- then adjusts speed of the fan to the conditions.

Also remember that most PA subs are not being used at below 40-45 hz.

Nothing down there, too much potential for sstage rumble, or runaway subsonic feedback, etc. Some systems don't operate below 60 hz.

Larry Acklin