Freq vs xmax

Hi there, ive been searching all over this forum and google but cant find either a solid xmax plot or a answer on my question.

I have 2x hd215's so a stack of 4 you could say. They are loaded with the Kappa 15LF - 8 ohm.
I give them 2500 watt @ 2 ohm. Normally i cross them @ 52hz till 128hz - 48db L/R.

Ive been reading that Dom used to set the crossover @ 40hz without problems.
But does this count for the xmax with the kappa 15lf too? or am i too paranoid?
i would like to set the crossover point @ 40 for just some extra more sub with dubstep and hiphop.
also notice that they will be given fair enough power so it might be dangerous to drop that low?

and when going as low as 40hz can they still handle till 160hz?

not much info on maximum crossoverpoints... and i dont mean what sounds good i mean what is safe before you are risking damage.

cheers, bart.

not till i have 4x X1 build to accompany those hd15's, no.
however, i dont mean this harsh,  i just want to know what the kappa can take not what sounds better of could be different or any other suggestion, just a pure scientific answer please :)