AMT drivers tend to be almost purely resistive, so even 40 years ago they would have been very easy to design for. Having said that, a crossover must take into accou t more than just 1 driver. Using Xsim and the right measurements of the two drivers that must be integrated is surely much easier and affordable. A $12 measurement m8c and Room EQ wizard or Omnimic and DATS will be the equal if not far exceed what was available 40 years ago.
I made a slight improvement in sound by replacing the three Heil side series capacitors with Mundorf EVO Oil Aluminum caps. I had to readjust the midrange presence and brilliance controls after that, they really opened the speakers up quite a bit.