No need for a tweeter at all, in my opinion. On music without a deep bass line, they sound terrific. Classic rock needs some low end help, that's why I'm investigating the optimal sub set-up right now.
There box is a resonance chamber, and there are a series of slots below the driver that enable the output from the driver to get into the chamber. You can read all about it on the Decware site. There are 5 white papers that detail the speaker's development. I'm happy with them overall, but coming from a pair of Klipsch Forte II's I definitely miss the bass output on some music. That said, on most jazz,,which I listen to a lot, the speakers sound wonderful. Imaging is dead on and the soundstage is good. I'm still playing with room position, which should help grow the soundstage.