>>If a good sub is part of your HT, why purchase nearly full range floor standers (M80's, M60's), if the best results for music and movies are obtained by crossing them at 60 or 80hz?
Very good question. If you think about it, though, the big difference between M80s and say M50s is not the bass, but more drivers to handle the higher notes. If you want to play at high sound levels in a medium or large room you need to move a bunch of air even for the frequencies above 80 Hz, and that's where the extra drivers of the M60 and M80 come in.
Let's say the sub covers from 20 to 80 Hz, or two octaves. The main speakers still need to handle the remaining 8 octaves, and there's a lot of energy in those 8 octaves.
Could you make a speaker with all the drivers of an M60 or M80, put it in a smaller box and sacrifice some bass extension, and still get as good a sound in an HT system ? Probably, but nobody would buy it
