Considering your speakers, I'd suggest that the THX researched and recommended 80Hz crossover should work well for all of them, including the M60s. There shouldn't be a good reason for setting the M60s large.
Actually, if you spend any time reading Earl Geddes, he makes a compelling case for running your mains full range (if they are capable) and having additional sources of reinforcement in the 50-10/200Hz range. People who have listened to his system rave about the bass response.
In a nutshell, Geddes has come to the conclusion that you want to excite as many of the rooms modes as possible to smooth out the bass response. He feels strongly that it is in this range that a lot of systems are lacking.
If it were anybody else, I would ignore him, but he has a huge background both professionally and academically in audio.
