Go the stereo sub route. This will increase the realism of stereo and mulit-channel music presentation. The added sub will increase low frequency extension by a few Hz by way of mutual coupling between the subs. The convention of a single mono subwoofer came from the motion picutre industry for movie playback - it was never created with music reproduction in mind. A mono subwoofer is an unacceptable compromise for music because all acoustic mixing in the room of the low frequencies in a recording is destroyed.