That's the big question I have. I've read a lot about the benefits of adding a quality sub to a two channel system. Apparently if pays off regardless of how well your speakers perform bass wise. The description's I hear are that it increases the soundstage in all directions, projects a better separation of instruments within the soundstage & just gives you much more of a feeling of being there. The only drawback is the integration, it has to be flawless to pull it off. Set up properly you shouldn't even know you have a sub running except in the musical experience. Here's a list of subs that I'm checking into. Feel free to comment on your experiences & recommendations.
Polk DSW Pro 500 - REL T1 or R 305 - ACI Force XL - Velodyne DD-10 - Totem Storm - Revel Concerta B12 - Vandersteen 2Wq - Sunfire True Subwoofer EQ.