The sub with the busted lead either exceeded xmax or had a faulty solder joint. Assuming the subs share airspace once one sub goes everything goes to shyt because you basically double your airspace for the good sub and the busted sub begins to function as a passive radiator. Once that happens, the other sub is doomed too because it's mechanical power handle will drop off dramatically due to the additional airspace. That's why many people build enclosures with separate chambers....
Gain looks maxed out on the amp - very bad. You were clipping and that gain setting has a lot to do with it.
Next, you have the bass boost cranked up ~6db at 30db, so you basically quadrupling power at 30hz, which leads to massive clipping and dead subs. Assuming the enclosure is sealed, you're asking for a lot of excursion from a sub that has a mediocre xmax. Again assuming this is a sealed set-up, why are running those subs? You could run subs in the ~3-500wrms range and get the same output, maybe even greater output, since many 12s would fit in the same airspace. Xmax determines output in sealed enclosures, not power handling.
Back to the massive clipping issue. So we've got the amp gain set way too high. We're asking the amp to quadruple power output at ~30 hz, which leads to even more clipping. I'm guess other bass boost features were turned up on the HU, which adds even more clipping. Then there is the fact that you ran the subs at .5 ohms, which pulled even more power from the amp.
I'd guess you smoked sub #1 with massive clipping distortion and way way way too much power. Somehow sub #2 survived this abuse as did the amp. Luckily for the amp, sub #1 died, which in turn reduced the load the load on the amp and possibly saving the amp's life. However, once sub #1 died, sub #2's mechanical power handling was compromised by the increased box size (not to mention the possibility that sub #1 was acting as a passive radiator tuned to a high frequency and you're boosting low frequencies) so it exceeds xmax and yanks that tinsel lead off the terminal. I'd call it use error and if they set the gain and bass boost, installer error too. As a matter of fact they should have wired the subs to a 2 ohm load, the subs would have reached xmax with that power and everything would be good right now.
If you're chasing output, then I'd rethink that enclosure and the subwoofer enclosure. Those subs in a sealed enclosure won't get louder than my single 10" MK1 Adire Brahma off 1000wrms in 1/4 the airspace. IMHO, you're (probably) wasting airspace and/or power. Get something like a singe 12" Exo (or look at Sundown, Incrimator, Sound Solutions) and put it in ~2 cuft ported. If you're running out of space, then look at Earthquakes SLAPs passive radiators. Just keep in mind subs unload even faster when using a passives vs ports, so you have to set your SSF really tight.
Additionally, you would have had massive clipping distortion from the amp and massive distortion due to the subs exceeding xmax. You've got to learn what distortion sounds like and stop listening to it. Distortion is bad juju.
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