Woofer Excursion Below Ported Cabinet Resonance

Might be helpful for us to know under what conditions you're
observing the problem.

Is it occurring all the time without exception??...or with
certain volume levels or with certain discs, material or
genres of music?? When did you first notice it and had you
undergone any other system changes before you saw the
problem?? What kind of impact is there on the sound or
system performance?? Is this problem on the sub listed in
your system??

Any info you could supply might help. Have you checked your
sub crossover settings, or changed your sub's location, etc.

Unless you were talking about this in general maybe?? Not sure I understood your question...