If the speakers are on a bookshelf then it would only make sense to have speakers designed for that pi space - against the wall. This doesn't necessarily have anything to do with being sealed or having a port on the front.
The ones I can think of off the top of my head are the Snell bookshelf with the pi space switch, the North Acoustics Spirit and the new Wilson Duette. Of course, the latter is 10k. The others are hovering around 1k.