I don't understand why you would want to put an 18" in there. What kind of music do you listen to? If you listen to anything where you want tight base, then you wouldn't want to go bigger than 10". Also, remember...if you want that 18" to be anywhere near tight and efficient, then you better have a big amp to power it. An 18" would be great if all you listend to was Rap and Big Base music.
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Schleprock, Schleprock...stronger than steel!