I know JL probably doesn't want me speaking for them, but if past behavior is any indicator of future behavior, I have a hard time imagining JL selling their Stealthbox for the X5 though any venue other than their authorized retailers. They are a staunchly anti-internet-sales company (
http://mobile.jlaudio.com/jlaudio_pages.php?page_id=111) and VERY picky about products being sold at MSRP, and I would imagine they will stick to their policy for this product, too.
If my guess is right, the only way to get one of these is going to be through your local JL Audio dealer.