GLUE BABY! I thought one of my subs were blown before...weird noises coming out of it...put it away for a while thinking it was broken, tested out all my subs again and they all seem the same and couldn't hear the difference.
I put two of the subs back into my car, one of them I thought was blown but I cant tell the difference now, they still pound pretty hard...I don't know how to explain it but what happened was, I had two subwoofers attached to one amp, the wires going to one of the subwoofers ripped off and the power went to the other subwoofer.
The amp is a 2400watt amp and they are 1000watt subs, however I think it is 300rms per channel, the subs are bridged to the amp, when the wire came off, I can only think the ohms rating changed and the power didn't surge into one sub and blow it to sh*t. But it did sound messed up.
Try listening to each sub at close to their peak power separately, you should be able to tell by listening to it...if there is a smell or the coil sounds scratchy it maybe blown.
Edit: Reading through it is possible that a screw got inside the woofer through a hole in the back or something, thinking about it the alpine I have has a hole in the back of the magnet and it maybe possible a screw was pushed and rattled into the woofer (the woofer was reverse mounted) causing the scratching noises. Take a light and shine it in there and maybe you might see something...I think I'll check mine too...lol.