so let me get this straight.
amp is rated at 1000w RMS, gains "set to 3\4". so how do you know the gain is set at the correct voltage of the headunits pre-out? did you read the manual? did you match it to the listed voltage from the headunit? did you use a multimeter and the gain setting formula to see how many watts it actually puts out? does not sound like you did?
a sub that should handle 1500w. lets see now.
i see the type x listed as 500~1000w RMS, do you see that too? and what? in an
oversized box too? is that supposed to mean something? because from what i understand, having a larger box than recommended (again, recommended sealed enclosure size is .65~1.25 cubic feet, about .5 cubes too big) lowers the mechanical power handeling of a speaker. thus, the spider is moving too far for the size of the box and power it is getting. that 500~1000 watt power rating means, larger box size = lower power suggestion, and vice versa.
so a .65 cubes, they recommend closer to 1kw. and the 1.25 range, they recommend close to the 500w range.
so it looks to me like your buddy is giving them twice the power then they are recommended to recieve in that specific enclosure.
happy days.