Help, CSS SDX 15" sub bottoming out!

A couple of weeks ago I finished building a sealed CCS SDX 15" sub. The enclousure size is 3.75cf (internal), the amp is ED LT1300 and a ED EQ2 eq.
After running my yamaha YPEO, the sub is caligrated with my radio shack digital meter. The sub is on a space of 4,500-5,000cf room. The sub sounds good on music and at moderate levels, good on movies too. The problem is when the sub hits around 100db or higher on movies such as explosions, it makes a popping sound (driver bottoms out). My sitting position is 9ft from the sub. The receiver LFE setting is at 0 and the output at -10. Crossover is set at 80, small on the speakers and calibrated with the sub at 75db. The volume on the amp is set at 10:00 and the phase is 0. I've run it with and without the ED EQ2 but no luck getting rid of that popping sound. I plugged the receiver and the sub to different outlets (I thought maybe a lack of current from one outlet was a factor), well, no luck. I don't believe the amp is clipping. The enclousure is made out of birch ply, heavily braced (check my forum on "first CCS SDX 15" sub made" here. The space from the back of the driver to the plate amp is about 4". Maybe the sub is not wired correctly? It's a D2oms sub. Thanks for your help.

I've read someone tested the LT1300 plate amp (don't remember the site) and it put out 1332 watts, so it seems to perform up to manufacturer's specs. The CSS SDX 15" sub has a Xmax of 30mm and it says 1000rms. I have the box stuffed with 3.75 lbs of polyfill (1 lb per 1 sf). I'm going to test it again with a heavy bass movie to see if the amp is clipping.

Ivisneau, I think you nailed it, a friend of mine came to eat in my house today and he is telling me the same (he told me before I read your post about a bad coil on your sub). He's handy doing electrical work and while testing the sub with some mid level bass scenes, he pressed the sub inward some with his hand and you could hear the popping (clanking) sound even more, he told me the same, that it's a bad coil. The amp did not clip at all during the heavy bass scenes. I bought this driver used and it seems it came defective, that sucks. I suspected it was the driver from the beginning.
Can a bad coil be fixed? Any way around that? The good part is I got a TC Sounds 3000 used for $300 a couple of days ago on Ebay. I plan to replace the SDX 15" sub with the 3000 by next week when I get it. It should be an upgrade (if it works!), on the same enclousure with the same amp.

Next week, when I get the TC Sounds 3000 sub, I'm going to take the SDX 15 out and test each coil individually. It sounds fine, only when pushed hard it starts making noises. After checking the measurements, it looks like I'm going to have to cut the bafle about a quarter inch to fit the 3000. The bafle is 1.5" and the driver is going flush mounted, so cutting the outside bafle .75" is not going to be easy at all, any ideas?