How critical is the quality of the interconnect cable from the amp to the subwoofer. At about what price, for 4 metres, does the law of diminishing returns set in?
Cables and interconnects tend to be a contentious subject. I've never had any complaints using cable from Radio Shack. However, SVS sells its BetterCables 5 meter (sorry, jumps from 3 to 5 meters) for $42. I suspect that is as much as you would want to spend.