A couple of days ago I was talking to a dealer and he said that all speakers benefit from adding a subwoofer. What's are your thoughts? 
The low frequency capability (listed as -3db) of most main speakers is generally where not much bass is going on and that's fine…thinking my Silverline Preludes with 3.75" (larger than the 3.5" listed, but still) woofers get any real bass at the 35hz point or whatever they claim is easily tested with a low frequency test tone…yeah…there is some sound at that frequency but really you have to get to the 50hz and above point to see where they start to lose bass output. This is fine with me though as they sound amazing, especially with the RELs I use with 'em…musical subs are exactly that and I put mine at around that 50hz point so they blend perfectly in my listening room at from normal to loud levels.
Wolf_garcia, I agree, there is not much activity at 30Hz.   Another story is frequency response in the room.  My room amplifies about 30-35Hz extending response a bit, but extension is less important to me, than quality of the bass.  My speakers play all bass notes pretty much even, free of resonances.  Attack and decay of the string sounds very natural and undistorted.  Widely used overhang motor woofers often have huge distortions reaching 5%.  My speakers are also  (judging by the size of the magnets) overhang, but don't have bottom suspension (spiderweb).  This suspension adds to membrane's weight, slowing response, and affecting linearity of the motion.

Should you use the same speaker cable for subwoofer as main speakers?  Seems like the answer should be yes.  Or is there cable meant for subwoofers?