I have designed a subwoofer system of (8) 18" subwoofers in an IB configuration to maximize low frequency extension without overexcursion. This particular solution requres an external miniDSP unit and the use of biquad or Linkwitz Transform equalization.
Without the filter the subs reach excursion at 15hz and will bottom out quickly after attempting to play anything below 15hz.
With the filter the subs can play with smooth curve all the way to 3hz without overexcursion as long as each sub receives no more than 90w.
There are other SSF options available, but this is what I would like to try first in the way of protecting the subs from overexcursion.
I would like to take advantage of the XT32 processing power of my Denon AVR4311ci, but am concerned that it might push the subs to overexcursion as the processing they apply is an unknown. Specifically I'm concerned that it might apply boost in the regions of 5hz-30hz that could allow the subs to bottom out on a deep note in a movie. Excursion in th danger region of 5hz-15hz could easily be affected by wide parametric boosts by XT32 in the 20-30hz range.
If XT32 applied only cut to peaks and did not apply any boost, this would be a moot point.
This concept really applies to most well designed ported subs as well, as boost applied to the low end could alter the effect of the SSF designed for that system.