The specific type of distortion likely to occur in a small but high-quality two-way loudspeaker driven hard is due to the woofer being driven beyond its linear excursion limits. Over-excursion results in what's commonly called "fartout" in the prosound world. Some motor designs go more gracefully into over-excursion than others, but eventually just about all of them will get there. In extreme cases, a woofer could be driven to its physical limits, which can result in the voice coil hitting the backplate with a disturbing machine-gun-like snapping sound. This causes damage to the voice coil and/or voice coil former, which may or may not kill the woofer right then and there.
As noted by sboje above, cone excursion quadruples for each octave lower you go, for the same SPL. This would be true for a sealed box - for a vented box, it's more complicated. The backpressure from the port decreases excursion near the tuning frequency, but then excursion rises very rapidly below the tuning frequency. So if the small monitor speakers are ported, the main danger zone may be lower than you would have thought, but also more severe than you would have thought.
In my opinion not only the highpass filter frequency, but also the highpass filter slope, comes into play if safely getting higher SPL out of a small speaker system is the goal. Steeper slopes do a better job. With all due respect to Richard Vandersteen, in this situation, imo first-order slopes may well be inadequate.