I would concur with chesseroo that you should pick up the M80's and give them a whirl before you think about buying a sub. However, as an M80 owner who uses his system for 90% music, I did decide to add a sub, and it has made a substantial difference to my listening pleasure. (the water cooler's little brother - more like a water barrel) Without being overpowering, it puts a "feel" to the ground floor which the speakers by themselves cannot give.

Ed