mtx thunder 8000

My sister has two newer model mtx thunder 8000 12 inch subs hooked up to the big mtx thunder mono amplifier in a box made by mtx that is 3/4in mdf.  Everything matches and is wired up correctly.  When the gain is turned up on the amplifier(even with the low pass filter turned down), the speakers flex way too much and they have a horrible sound.  The sound is hard to explain, but i guess it sounds like a nasty horn.  Does anyone have a clue about what is wrong?  Are the speakers blown?

well for one thing, the thunder mono amp wont do the wattage for the subs. (or i could be wrong) but my suggestion is try hooking it up to a bigger amp. if your friend has a bigger amp, or you can just take it to your nearest car stereo shop and just ask them to hook it up. cause it seems to me that the prob is with the amp. and not the subs and i think the subs are distorting as well. if that doesnt help, reply back! xTimx

I hooked the subs up to a 2400 watt amp and they acted the same way.  I know the amp is fine, but the subs are still distorting real bad as soon as i turn the volume up past like 5 (not very high).  I also noticed that the distortion was a lot worse at higher frequencies.  Is there any way the problem could be the box or anything other than the subs?