If you want pure bliss of 2-channel , the sub will be expensive.
There is a speed factor or lag thathas to be in limit for the lowend filling
The bass part has to be covered 'in time' .
Most people miss this part and go by thump power or LF extension of sub .
As per as movies those are real specs but for music the sub has to fast so
that it fills the low end right in time.... such subs are little expensive .
So moral of the story , dont buy sub unless you are not sure if the target is 2 channel stereo , think why lyrithaaudio people did few months Rnd to add a sub to their speakers