Maybe you're missing something in the protection circuit that's preventing the amp from shutting down when it's abused. Have you been checking the small inductors that feed the protection circuit drive to the driver board?
I've found that it's difficult to pick subs for class D amps without thoroughly testing them. There are too many variables to be able to know which ones are important to reliable operation. This also applies to other classes of amps. Two examples...
The IRF540 works in Rockford class AB amps but the IRF540N are VERY likely to cause problems.
The IRF3710s produced years ago worked in the 4080 based amps but the ones that are currently available will not. The IRF3710Z has to be used to replace the older IRF3710 (non Z).