subs are more to match a room than a system. the lower bass has a basically nonexistent "timbre" or "voice". you don't have to buy (for example) an Axiom sub just because your other speakers are Axioms- the front and center should be from the same manufacturer's line , the rears not so much, but the sub can be completely free-lance.
the adjustable cross-over on pretty much every sub out there will allow the sub to take over where your mains bottom out.
several good units include Outlaw Audio, Velodyne, HSU, and SVS.
just get a unit that fits a) where you can put it, b) is as big as you wife will let you have, and c) can produce enough bass for the size of the room. most sub websites have room-matchers that let you enter a room size and they recommend a sub back.
when you do get your sub, go to the set-up tips on this website and read the placement on subs. there's really good info there.
http://www.audioholics.com/techtips/setup/loudspeakers/subwooferplacementguide.php
