Hi @unreceivedogma,
I use Altec 604E with original and Werner Jagusch crossovers.
The horn of 604E compression driver has an issue - ringing on 7.5KHz.
Mastering Labs crossover filter out this frequency. It also suppresses upper bass and lower mid and increases mid and low bass. I never heard Mastering Labs crossover.
Werner Jagusch crossover suppresses high frequencies from 5KHz and up. So sound become very smooth but very dark. It partially can be compensated by external super-tweeter.
The original N1500 sounds bright when high frequency regulator is on maximum. But with some high frequency regulator adjustment -3.5dB (7.5 Ohm on part of potentiometer that serial with compression driver) the sound become well balanced. In this case external super-tweeter helps to get more refined sound. I just built this crossover using original inductors, good quality capacitors and constant value resistors. I like n1500 more than Werner Jagusch. n1500 gives a better more focused soundstage, flatter more neutral response on high frequencies. Plus original n1500 inductor give more controlled bass.
I use Fostex T90a supertweeter. I used it with 1st order crossover (like 99% people do) for 1 year and moved on to 4th order crossover, that give me a huge improvement.
Regards,
Alex