i currently have a par of Pioneer - 6" x 9" 2-Way Coaxial Speakers and Basalt Cones with a KENWOOD-KDC-4054UB head unit, i want to attach a sub woofer if the back of the landrover defender and was wondering the best way of doing so would this http://www.amazon.co.uk/Boss-Audio-Systems-BASS1500-Shallow/dp/B0032FOJXE be useful or would i be best getting a sub and amp separate, or even a new head unit? please help!