This is the Bose Acoustimass series 2, essentially a Bose Acoustimass series 1 but with a horizontal centre speaker over the standard vertical of the previous model. The Bose Acoustimass series 2 module offers 6.1 as there is a centre rear output. This is a gimmick really, all but useless, unless you have the extra speaker and a 6.1 amp, even then its not that good. Bose Acoustimass series I and II bass units are interchangeable. These bass modules are huge and consideration to room size and placement may be needed. These break down regularly. Bose have discontinued this series now so becoming rare, great units, booming bass.