Joe, regardless of the pretty good bass response of your M60s, a good sub handles the lowest bass even better and relieves the M60 of some of the lowest bass burden. Set them "small" with a 80Hz crossover, which is a good compromise frequency, being high enough to take a significant load off of the speakers, but not so high as to make the sub easily localizable.
Incidentally, several receivers beside HK(e.g. some Denon and Onkyo models)provide for setting different crossover frequencies for different speaker groups.
