It should be possible.. Just take a look this system with a somewhat similar power consumption. http://www.silentpcreview.com/Puget_Com ... CR_Edition
It uses the Seasonic X560 that receives no airflow in the lower chamber. It is run completely fanless since the power draw doesn't reach the point where the fan starts to spin. If that thing can be run fanless in a quality built like the Serenity Mini, then the Seasonic x400 should be able to run in the lower chamber, especially since it has just a bit amount of airflow.