The difference is that with the M80s you CAN set the crossover lower, whereas a bookshelf 80hz is pretty much it. I found I get better mid/upper bass running my M80s at 40hz set to small to let the sub handle the rest of bass. If I have the M80s set to large they play down to ~30hz along with the sub and the bass can become boomy sometimes.

The reason to set the mains to small is to keep the lowest frequencies sent to the sub and only the sub, this prevents 'double bass'. There is no rule set in stone here, you can set any system up any way you like. I have found some rooms can benefit from the mains and sub doubling up on the lower frequencies to help with nulls.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70