Rule of thumb when ducting any machine that normally has free air flow in and out, this applies to chillers, air compressors and and other self contained machine using high flow fans such as miners: Every single manufacturer I have worked with always required a powered ducting system rated at a free-flow equal to the machines stated airflow. Failure to do so voids the warranties.

Don't hold me to it but AFAIK for a s9 that is a free flow in & out of around 250 cfm. Considering the fans are already working hard to keep reasonable temps, raising the work load by adding adding resistance (passive ductwork) will only reduce the miner's internal airflow and result in higher chip temps.