This sub does not have a pole vent on the back, which I do find a little odd.
I think the whole X line of Sundown Audio does not have a typical pole vent.
There is a spacer between the motor and the frame, the spacer is plastic and has straight cutouts as ports evenly spaced around the diameter.
I wonder if they would have gone with the ct sounds design of using metal cylinders as the spacers, rounded edges are better than straight ones when it comes to air noise.
http://ct-sounds.com/wp-content/uploads/2014/04/Meso8_banner_1140.jpg