The bargain basement prices that you are finding on the Def Tech and Polk are undoubtedly come from grey market suppliers. There will be no warranty on them. Also when you buy from grey market, you can't even be sure that you are going to get a 110 volt model.

I have heard several sized of Titanic and have liked them all. I haven't heard the Sub 100. While the rate the same, I think the Titanic will perform much better. It has a more powerful amp and a much beefier driver. That being said, I've never had/heard a Dayton product I didn't like. The bang for the buck is outrageous on them and they have a 45 day unconditional satisfaction guarantee. You seem to be favoring the lower end of your price range. Accordingly, I would try the Sub 100 without giving it a second thought. Worse case scenario you can return it.