Sub Sonic Filter setting?

You want the ss filter to start cutting freqs below the box's tuning frequency on a steep slope.  The tuning frequency defines the frequency that will be played primarily through the box port.  Below that, there is no damping effect on the cone, so if there are strong signals in those low sub regions the cone will move in and out uncontrollably.  You don't want to set the ss filter ABOVE the tuning freq because you would lose some of the SPL created by the box, so you set it below it.  Just keep it as close to the Fb (tuning freq) as you can adjust it.

When using a vented enclosure, always extend your initial setup to include the most precise testing that you are capable of.  I would play a sequence of sub level test tones and watch the cone movement while adjusting the ss filter to keep any uncontrolled flapping in check.