I also run my Maggies (MMGs) through a B&K EX-442 Sonata. I’m using a Rythmik F12G sub. After lots of back-and-forth, I settled on 80 Hz for the crossover (it was either 60 or 80 for me). With the room EQ/DSP I run it integrates very well when the distance setting is done right. My PLP is about 14 away from the speakers so I need to add distance to my sub setting (about 7.3 feet to be exact). I’ve found it’s largely a timing issue. Frankly, I don’t know what a 50 Hz crossover and real low gain is going to do for you. I’ve tried it and it didn’t seem to add much to my listening experience. YMMV.
Clearly, I'm in strong disagreement with those who say you can't successfully integrate a sub with Magnepan speakers.