As an added reply, I should have noted that the Ultra-X12 being a downward firing sub with a downward firing ports, caused my concern. The question of 'loading' really applies to both the driver AND the ports.
My KEF LS50 main speakers are vented, with the port being on the enclosure rear surface. KEF makes a great fuss about placement of the speakers with respect to the enclosure to wall distance, and includes 2 port 'tuning' plug sizes to compensate for placement distances less than 500mm (about 20-inches). With that in mind, I am as concerned about BOTH the driver and port distance from the floor.
Thanks again for your interest.