When the sub sounds like a basketball bouncing when the bass hits sometimes, the damage that's caused when the sub finally goes dead by that action, is that fixable?
What subs are they? Some may be worth reconing (which is the replacement of the cone, spider, leads, and surround but not the motor, basket, etc.) while others would be just as easy to replace entirely.
One more question to this, is that sound more due to the rms/voltage exceeding the subs capability, OR the power not being a perfect sign wave/clipping...exceeding the amps published rms?
It sounds like you may be reaching the mechanical limits of the sub from a improperly designed box or giving it too much power. I've never exactly heard a sub sound like a basketball...