u got screwed by whoever sold the subs to you. The coils are a little fried or if you got lucky the glue just got hot, the suspension needs to be firmly glued to the cone. When testing with a DMM make sure to measure based off manufacturer specs and go with either lowest rating for rms whether it be the amp or the subs. Take time to make sure you have proper electrical for your system, you shouldn't need to upgrade much for 800wrms, but thicker wire might help. And definiately set your gains according to the DMM, turn all bassboost of any setting down. Subs should be filtered at around 80hz. Check voltage from batt while system operates on heavy bass tracks, make sure you don't drop below 12volts.
2 scenarios are possible.
1. You are overpowering the subs with clean power (i'm doubting this though it is possible with that amp). Fix=Setting gain with DMM.
2. You are getting clipping/distortion due to electrical supply/power issues. Fix=turning off all bassboost, turning amp gain down till you are no longer dropping below 12volts.
The best way to prevent damage to your speakers/subs is to have a proper electrical supply to your amp, set gains properly, setting filters properly, and not boosting frequencies at the lower end of the range.