The biggest flaw of almost all subs is the lack of a high pass filter for the main speakers. It's hard to do when you don't know the main speakers. But properly done a high pass will make the subs/mains twice as good because the main woofers have to do orders of magnitude less movement resulting in instantly recognizable better sound in the new bottom octaves of the mains. So I'd go for a sub with a high pass or find a way too add one your self.
Also, this is sort of taste. Closed box woofers are more accurate than ported ones(or passive radiators). But they don't sound as rich. Closed box subs are rare. Rhythmic makes them. There must be a few others.