I hope someone has a more direct and productive answer to Markus1299's question.
I have owned 4 sets of Magnepans over the last 30 years or so, and although I have never modified them, my understanding is that both upgrading the passive crossovers and biamping can yield significant improvements.
In the case of an active crossover, perhaps the quality may not be as good as the best passive, but it enables biamping obviously and potentially huge amounts of power.
Many Magnepan enthusiasts might argue this improves sound more than incremental improvements in the transparency of the crossover.