Hello,
First thank you guys for sharing all the info here regarding the Mastering Lab crossover. It was very helpful for me in my attempt of recreating it.
I was able to come across a great pair of Super Big Reds - the ones with a bass helper. Unfortunately they came without the original ML crossover - but only with the Altec N-1500 crossover. I read everything I could find about various crossovers and made several attempts in making different iterations of original 604 Altec crossover. I read somewhere that someone said that Altec N1500 crossover is excellent crossover but needs something to be done to tame shouty mids. And I completely agree with that statement.
But I really wanted to try Mastering LAb's crossover and I did it. As we can see from conversation here there were some unanswered questions about the orientation of 1.5 mH coil. I will post here measurement so you could all see the difference. I could say with the absolute confidence that bigger portion of that coil goes toward junction of 20 ohm resistor and 2.3 uF cap. Not vice versa as it was shown in this schematic posted above. Here are the pictures and measurements. I did electrical measurements ( not acoustical ) on AP with the crossover and speakers connected. I measured it both ways so you could see the effect. I also measured effects of the pots so one could see exactly what and how much they affect the sound. At the end I am really happy with this crossover. It addresses very successfully the issue of over pronounced mids and shoutiness.