are you just talking about getting a subwoofer? or do you need tweeters and supertweeters too? I guess the third question I have is about how many... do you want to have a single sub setup or dual subs (if you just mean subs)?
When I've gotten speakers before,
http://www.crutchfield.com/ has been really useful. They provide lifetime support normally too. Another site I've used since they may have stuff cheaper is
http://www.sonicelectronix.com/ but they don't have the same support. A lot of the reviews from Crutchfield are very helpful and actually are worth something. I'm not sure about those two websites for ordering though since you are in Norway.
A big factor in choosing a sub is the amplifier powering it though. I know you said to not factor it into cost, does that mean you already have one? If so, which model is it (so we can match a sub to it). If the amp has more power than the sub is rated, you'll probably blow the sub pretty quickly. If the sub can handle way more power than the amp is pushing, you won't reach full sound and the sub will sound punny. If you don't already have an amp then the amp should just be matched to the sub while considering the cost the entire time (in general, the higher wattage the sub is rated for, the more expensive the amplifier is going to be to power it). The type of amp also matters if you were going to try to throw two subs side-by-side or if just using a single sub.
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