well, i would suggest single pass.
double pass doesnt sit right with me. it means cooling water in one direction, and bringing it back up to the top again... where the hot water is going in. maybe not directly beside the hot water, but it means you're weaving cold water between hot water. i highly doubt that double pass is as efficient because there is plenty of time for heat to be conducted back into the cold water... not much, but the fact still remains.