Hi Clayton,
Well, honestly, you can't really expect an 8" sub to go down to 20Hz....30Hz is probably more realistic.
There is no "fix" for port noise. The only thing you can do is not play the sub below it's tuning frequency.
Is there a subsonic filter on that sub? Usually, it's selectable/defeatable. If it has one, turn it on. The designer of the amp/sub will typically set the frequencies on the subsonic filter to coincide with the tuning of the cabinet to ensure the sub isn't played below is tuning frequency, thus preventing damage.