I guess the JL Audio box is amazing. Here is what they have to say about it.

The fiberglass enclosure offers superior rigidity while allowing more interior volume, so the sub can turn out louder, deeper bass. JL Audio craftsmen then laminate, grind, join, and finish each Stealthbox® by hand.

So the fiberglass gives them a little more volume. I am so close to pulling the trigger on this one.

Did anyone out there install it without modifying their cover? I really don't want to make any permanent mods. That is what is holding me up on this one.

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[Sold] 2004 X5 4.4i - Sterling Gray - Truffle Brown, Sport, Preminum, Rear Climate, DSP, Sirius, PDC, 132's (19 inch wheel), Comfort Seats
Dynavin, JL Audio Stealthbox, Alpine PDX-5 Amp