I've read a few things pointing to the crossover in those cabs as being the weak link. Having had a 1516 and an 1810 through both with bad crossovers kinda points that way too but me and my 2 cabs does not a consensus make. Also BW18's don't have a whole lot of xmax, I don't know what xlim on them is though. I do know that when you model them, they want a fairly big box and the displacement limit gets quite low.
If your tech is a real tech, he should be able to put together a 2nd order Linkwitz or something comparable out of raw parts without much trouble. That would at least take the crossover problem out of the equation. He should also be taking your blown baskets aparts to find out why they're blowing. Did the coils thermally overheat or was there mechanical damage/shorting anywhere.
