They don't just "cut off" there is a roll off or "slope". Do you understand how low 80hz really is? The reason each speaker can be set with different crossovers is because different speakers have different frequency response. You don't want to send them a lower frequency than they can handle efficiently. If you study the +/- 3dB point for each speaker you will see that they are different.

Google is your friend.

It does not lead to gaps, not sure why you would think that way.

M80s VP180 4xM22ow 4xM3ic EP600 2xEP350
AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85