I'm not entirely sure, but I think some subs require the pressure a smaller enclosure provides to work properly. If I were to take my subs out of the box and set them on the table, they wouldn't sound right. My subs need pressure behind them to work properly. Basically what a sub does is move air. If there is no pressure behind the sub (like from being in a box) The sub looses excursion length (how far the cone moves back and forth) and you lose some of the air moving qualities of the sub. That's why it's important to properly tune a ported box, if the ports are too big for that sub, then it wont sound as good. That was the problem I had. My old box had almost 2.5 cubic feet of air for each sub and it had two 5.5" wide ports on each end. My subs were not designed for this enclosure and therefore sounded about as good as a much cheaper sub. I was missing out on the tight bass a smaller box would have provided. If I'm all wrong, dnewma04 will correct me, but I think that's right.
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The Honkeytonk Monkey
Jewel Green 96se MTX