well if it didn't blow all the way as it hits more the coil move thus causing it to hit that short that could be your problem,
i'd find a buddy with subs that take more power and hook those up and see if its the amp or sub
also if the amp dose it on my advice i would say it melted a wire inside the amp before the fuse blew and its not touching put with the more push it grounds out
thats IMO