DK, the point isn't whether the M60s are "capable" of handling 60Hz or lower; they certainly are. The idea of using bass management with a good sub is that the sub can handle those frequencies even better and can relieve the speakers and the receiver amps of part of the low bass burden, allowing them to do a bit better job with the higher frequencies. Tests by THX and others have indicated that 80Hz is a good compromise crossover setting, being high enough to take some load off the speakers and amps yet not so high as to make the sub easily localizable in most cases.
