like everything...it depends. depends on the room, the speakers, the music you listen to, etc. its a fucking bitch to correctly phase a sub with speakers though. you want it to be coherent. you shouldn't even know that there is a sub in the system unless you shut it off.
however, no matter what, any bookshelf will NEVER have enough bass impact/slam for heavy metal, large scale symphonic shit, kickdrums thru the chest, etc.....if that's what you crave. You can get it to be satisfying anyway.