overpowering subs?

...And over-excursion occurs before it gets to the point of bottoming out.  There is a defined amount of play (excursion) which you will find reference of in the parameter notes.  Say, for example, it has Xmax listed as 17 mm.  Look at that distance on a metric ruler and you will see what it's playable limit of one-way excursion is.  With your hand, push the cone from its resting position.  It will be within the boundaries of the specified Xmax if you push it back to any point up to 17 mm.  The cone can also extend forward from resting position that same length, safely.

While playing, you will see the cone move forward and backward equally (or very much so).  The excursion you see will appear as Xmax * 2.  The sub with a specified Xmax of 17 mm will appear to move within a range of 34 mm (17 mm * 2) and still remain within its safe excursion range.  That translates to a total range of 1 1/3 inches and is the distance you will judge when looking for over-excursion.  Figure out the total excursion range for your sub based on its Xmax.

But it will go farther...but then will be out of boundaries and precision will be lost.  This longer area of one-way excursion is noted as Xlim, or the limit of excursion.  It allows the sub to stay in one piece when encountering the occasional thump that might occur in a soundtrack.  It is also there to allow for some amount of the excursion problems that come when the sub is playing tones below the box tuning frequency and those lower freqs aren't being adequately filtered.  You really want your sub to stay out of that area, though.

So when I said you should be watching for the overexcursion, that means you should make a note of how far back or forward that sub can healthily go and watch for out-of-the-ordinary wobbling or fluttering.  This will occur in a vented box if all the setup conditions aren't being met properly.  Those conditions include (possible) use of a subsonic filter and extra care in preventing an overpowering situation (which you can use the amp's gain for).

Take extra care in the setup by playing recorded tones at lower freqs to see if you have filters set correctly.  If the sub has to play those tones that are well below the box tuning frequency, it will lose control of its excursion because the box has lost control over the damping of the sub.  This is where you notice suddenly greater amounts of excursion and out of control movement that I call "flutter".  At the same time, you don't want to set a subsonic filter higher than it needs to be set because you could lose some of the low freq range your vented box may be designed to enable.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.