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.
I've been using the DD+ 10 for my computer audio system for almost a year now - it's easily the best sub I've had there. TIght, with excellent control and speed, And since the controls are within arm-reach I, too can tune in some of bass shy or bass heavy recordings...
Steven Stone
Contributor to The Absolute Sound, EnjoytheMusic.com, Vintage Guitar Magazine, and other fine publications