I was always hesitant about getting a sub for my main system. That is until I read a post by Paul McGowan of PS Audio. In it he stated that he was always bemused that people had reported problems trying to integrate a sub. He recommended a servo controlled sub for smooth integration.
My mains are Focus Audio FS 888's and in a smallish room (12x 20x 8 foot) I went for a Velodyne DD-10+ and can report it is just great. Easy to set up, it even EQ's for your room. The remote has multiple controls that make it a breeze to adjust the large variety of recordings I have. It is really good to adjust the bottom end of some recordings with questionable EQ.