At first I thought that this sounded crazy, that you are having this problem in a small sealed room, like you have. But, if you listen at 75 dB, for example, then the soundtrack can call for up to 105 dB peaks from the subwoofer on a movie soundtrack. The most detailed measurements of what the F12 should be close to in terms of measurements are here:
http://www.hometheatershack.com/foru...ealed-56l.html. Many modern soundmixes for action movies contain a significant amount of content below 40 Hz. You should get a lot of room gain in this room to extend some of the curves in the long term output compression extend flatter to maybe even below 20 Hz, in your room, or maybe even be slightly elevated as you go down into these lower frequencies. But, it is probably not likely on the 105 dB sweep when you get down into the low 20 Hz range. So if you have peaks that are surpassing 105 dB it seems reasonable that you might bottom out the driver. The problem is even worse if you are usually listening at 85 dB (reference volume). How loud do you listen to your movies typically?
If you are not listening soundtracks at anywhere near 75 dB, 80dB, 85 dB and up (on your receiver volume dial), then I think that Johnny Mac is right and you should call Rythmik. If you do listen that loud, then you likely need a larger sub. Just don't surpass 85 dB as a usual volume level, since you are then bordering on possible hearing damage.