The sub handles only very low bass energy below 100 Hz. There's still lots of upper bass energy from 100 to 800 Hz that the main-channel speakers must reproduce, and larger main speaker's woofers and midrange drivers will exhibit greater dynamic ease on complex movie soundtracks, especially at high playback levels. Larger woofers simply have lower excursion (back and forth movement) than smaller woofers to move the same amount of air. When the large woofer moves less, distortion is lower, and it can handle dynamic peaks without strain. It's also a function of the volume of the listening space, not just how far away you sit. Even with a sub, the M60's will run louder and cleaner with less distortion than small compacts would in a room of a given size.
Axiom Resident Expert