not sure about designing boxes :-/ i guess you have to put the params in winisd (make sure you have the pro vers) and then design a 4th order, without changing anyting...
i'm also from europe, but that won't stop me from ordering a sub from the usa
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if you're in UK, you can try
www.bladeice.com for re subs, but they're kinda pricy. pick up any good 10 sub. try pioneer or infinity. if i managed to find re audio, adire audio, pioneer, infinity and jbl in romania, i'm sure you too can find those in the uk, and even more