Jordan, since the basic idea is to allow a good sub to handle what it does best, even mains with pretty good bass of their own, such as M60s, should be set small. Partially relieving the M60s of the burden of the lowest bass allows it to handle the rest a bit more cleanly and does the same for the receiver amps(actually this is a form of active bi-amping) . The THX recommendation of an 80Hz crossover has been found to be a good compromise, being high enough to relieve the mains, but not so high as to make the sub easily localizable.
