It does make a difference where you locate the filter. Keep in mind that if you use a surge tank (as I and many do with their ET's) that the tank is filled with fibrous filler. It is possible for some of this, and possibly contaminants (oil?)from the pump itself to dislodge and work their way to the air capillaries of the arm. So, it seems to me, the best place is AFTER the pump, and if using a surge tank, AFTER that.
