I am quite interested in if you can actually hear any difference between the M80 with a sub and without when playing music. Looking at the graph
The M80 speaker doesn't really have much db falloff until you are around the 30-35hz range and that is getting very close to the inaudible range anyway. Unless you are listening to a highly processed dance/rap music that uses the synthetic boom lower thumping sound, I can't really see much music getting to that level.
So I wonder, in setting up your crossover to limit the M80 to lose below the 80hz where they are clearly capable of playing down to 30hz is hampering the speaker to do it's job.
