Othen more cost and a more directional sound, there is no huge advantage/disadvantange to running M60 or M80 for surround duties, unless you listen to alot of multichannel music or action movies where you may find a bit more impact from a direct radiating speaker at the surround channels; many use M2's for rear channels in 7.1 systems.
The idea with the QS series is to mimic the array of speakers found in a theater. They diffuse/spread the sound around you, more enveloping vs. shooting the sound right at you. If you have a fair bit of space between you and the speakers, direct radiating speaker can work quie well, but in most cases there is insufficient space for proper dispersion from a direct radiating speaker so its sound is more pin pointed rather than enveloping like a speaker array or the QS series.
