Hi aldom,
You should spend considerable amount of time reading reviews about the subwoofers and going for multiple audition to get the idea of how a sub would sound. Every dealer would suggest you a sub-woofer that would have impressed them or in stock but need not impress you.
So take time, review every sub-woofer, understand its concepts and terminologies, compare products, read reviews, if you aren't familiar before. Most of the time, a buyer takes at least a month or two to see various sub woofer and finally settles down with one, even two.
Don't just follow what the dealer suggests you. Audition as much as possible and decide. If you have questions, please ask; we are here to help you.