Duke worked with a designer at Eminence to get the characteristics he was looking for at the time. He had to make a minimum purchase in order to get them, and overbought so that he would have spare parts.
I played for three years with Bam Bam the drummer, driving my v1 TC112 hard with an MB800 and then with a Streamliner 900, and never had a problem. I'm still using mine weekly at more reasonable levels, with an MBFusion 500.
Still, there were reports of some people popping the Deltalites at high volume, so I wouldn't be surprised if Duke is out of spares for at least some of the configurations.
IIRC, there were ways that he could re-tune the crossover to make a substitution work. There would be a loss of performance, but not a huge one.
Just makes me more protective of mine...