Magnepan claims 50 Hz. I have found their claims accurate, but as with any dipole you have to play with positioning to optimize the bass. Furthermore, distortion rises with large excursions that LF's generally exhibit, and you are giving up dynamic range, so I would cross them maybe half an octave higher, around 75 - 80 Hz.
I have never heard the MMGs. I had MG-Is, which appear similar, and crossed them at 50 Hz for a while. They were much happier crossed a little higher (75 Hz) after I built a sub to take over, and the system was much cleaner, louder without the panels trying to reproduce bass signals.
Thank you, I will cross them over at 80 then that way there is no worries, will the Classe CA-5200 be sufficient I read that hey we're very power hungry!
On an other note it seems that Magnepan does not respond to email!! Not very customer oriented?