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The site that I found about that amp is a bit funny, but as I understand it, amp can do 1600+ W @ 2 Ohm.
So you should look for two 4 Ohm SVC subs, or two 2x2 Ohm DVC. This way each sub will get 800 W. Which brand is best? There isn't such thing as best sub, there is a lot of good subs. Go for some reliable name which is in business long time. Go for good support too. Sub can't perform well without good box. Seller should be first person to give you reliable info. As I said, I shouldn't talk about my preferences and experiences, but then I haven't try all of them (hint: I am moderator, so I shouldn't talk about brands, but members can). So choose good and reliable brand. Maybe you can ask that question also on subwoofer part of this forum.
What do you mean by: Buy in USA and put in car in Germany? Do you want to buy that in USA e-store and import that on your own? That is legal and possible. But you will have to pay transportation cost, custom duties and tax. And you will be (almost) without any warranty. Is it worth it?