I am also glad I don't hear this compression.
JL use a 3.5" voice coil it should not normally compress too much at high
levels.
However, it appears to be a long coil short gap design which explains the high
harmonic distortion (around 10% and sometimes above). The long coil short gap
would be typical of PA and rock concert woofer designs - some manufacturers
make similar drivers in two versions (one short coil with less SPL output but
better accuracy and one long coil for more SPL) perhaps JL113 may lose power
due to very high excursions (4" peak is an unbelievable excursion- WOW!)
as the coil moves well outside the linear magnetic range, which woudl explain
the compression and THD. There is no doubt it is an excellent sub - one of the
best out there.