RD, the large/small decision isn't based on the physical size of the speakers involved and to a large extent it shouldn't even be determined by the ability of the speakers to reproduce the lowest bass frequencies. The idea is that a good sub should do a better job with the lowest frequencies than even a speaker(such as the M60)which has good bass. If it doesn't, the sub hasn't been set up correctly or simply isn't good enough compared to the mains.
The receiver doesn't send "everything" below the crossover point to the sub, but rolls off the response of the mains below the crossover point(nominally at 12dB/octave)and rolls off the sub(24dB/octave)above the crossover point. Tests by THX and others indicate that the best compromise crossover frequency is around 80Hz, being high enough to take a significant part of the load off of the mains and their amps(a form of true biamplification)and not so high as to easily localize the sub. However slightly higher crossovers at 100-120Hz are usable if the sub has good response there and localization shouldn't be a problem if the sub isn't located in a different direction from a main speaker. Note that the response curve on your PB-10 shows smooth and flat response up to about 200Hz, so that factor shouldn't be a concern. Set your M60s small, don't worry about the crossover much, and enjoy the music.
