if it's designed for free air it already has a high Q. if you put it in a box this will only raise the Q higher, making it 'boomy' and empty sounding. I'm not sure 140 watts is enough to power that sub. check to make sure that you aren't getting back wave cancellation. make sure that the rear is sealed off from the front by at least 9 feet (so that the wave has to travel 9 feet to see the front)
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