sorry to hear about your drivers guys.
here is 1000 "watts" through the si18 in a sealed enclosure.
the actual current through the driver dips around 40hz because that is where the resonant frequency is in a smallish sealed cab. at that point, current is minimal. then it begins to rise again, but an 80hz low pass filter then reduces it down and by 200hz, it's pretty much zero.
as you can see, down under 10hz, the driver is almost purely resistive. in other words, something like 99% of the energy going into the coil must be turned into heat. at 1000 watts, that is about 990 watts of heat. pretty much like running a hair drier and putting your hand over the end so that no air flows. it will heat up REALLY FAST. unfortunately, most hair driers have thermal circuits, subwoofers do not.
that test track is a stupid test track. it runs a constant signal starting at 1hz and going up from there.NOBODY SHOULD TORTURE THEIR SUBWOOFERS WITH SUCH A TRACK.
this is just an extended duration heating of the coil until it blows. i'm not surprised at all, though i feel for everybody who didn't know better.
the spectrogram doesn't even look like content. it is just full power single digit torture.
the s.i. subs may be chinese low quality drivers, but this track is just abuse.
as for the berry output, 1000 watts is probably about what you were seeing.