Nhan, the idea of having a receiver with good bass management and using a separate sub is that the sub can handle the lowest bass frequencies even better than a speaker(such as the M60s)which has good bass. In addition, it can be positioned(e.g. the corner)for better bass than the mains. This isn't "wasting" the mains or other speakers, but rather allowing the sub to handle what it does best while taking part of the bass load off of the other speakers(and the receiver amps driving them). Generally all speakers should be set "small" and the 80Hz crossover has been found to be a good compromise, being high enough to take a significant part of the bass load while not being so high as to make the sub easily localizable.
