Adding a sub affords the opportunity to use digital room correction in the bass. Units are available from Audyssey, SVS (using Audyssey technology), and Velodyne (my choice). Cross at 100hz or so, and you can clean up most of the destructive room effects. For me, this is the primary benefit of a subwoofer. Just MHO. You might also need/want an external active high pass unit like the Marchand or NHT X-2.
Marty
Marty