As I have never had them before I thought I better ask the question before I start spending money. I have a pair of B&W 685 speakers and wanted to get a subwoofer unit, do I need to get the same make/brand ie B&W to match them with or can I match them with different brands? if so can I get a subwoofer on the secondhand market to test the water then I can make up my mind going forward & which kind would would well? Thanks