JL Audio f112 blown speaker?

This is an amplifier problem or power supply problem in your sub. This is not good - especially if two units have done this. Perhaps JL have developed a QC problem with some parts they are using. Alternatively, if your Classe has a fault perhaps a high level DC or transients are reaching the XLR input on your sub and damaging the input buffers by over driving them. If your XLR cables are wired correctly none of the three wires should be connected to the connector or shield and all three lines should be isolated from eachother.

After sending in my subwoofer to JL Audio they tracked down the problem to a preamplifier component in the sub. I beat the odds and got two of them with the same exact issue in a row.

Sounds like they changed supplier or got a bad batch. These things do happen even with great products. Funnily enough though - the HT Shack Subwoofer tests reported another problem with the JL sub they received for testing - so this incident may not be as isolated as it seems - check the HT shack subwoofer test forums for details. I don't remember what it was - I seem to remember they observed significant audio compression below 40 Hz at high SPLs..

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.

JPL,

4" is peak to peak - so 2 inch in and 2 inch out. This is the absolute maximum - it would probably sound even more distorted than 10% THD at those type of excursions.

Do we even know what excursions were taking place when this distortion was taking place?

The HT Shack Subwoofer test graphs are colored with a variety of measurements at various SPL outputs, all at less than insane levels. Many of the curves show what could hardly be called insignificant amounts of distortion.

If you look at an equal loudness contor plot of our hearing sensitivity then you will observe that the sound of 10% distortion will be much louder to our ears than an orignal 20Hz tone => because of our lack of hearing sensitivity at 20 Hz you hear the distortion louder than you hear the music.

Refer to the bottom of the page (last two plots) to see what Seigfried Linkwitz has to say about distortion at low frequencies.

Linkwitz suggests that 1% THD is the kind of distortion you ideally need in a sub - if you want to hear what is on the recording.