You understand that the speaker is only 4 ohm at certain frequencies, and in fact much higher for most of the range?
What we are saying is if one of the speakers are bad, which is unlikely, this way you will know this. If the amp shuts down only driving one speaker, and not the other, you will have your answer, I don't really care what the multimeter is talling you.

Most likely, your amp just can't handle the m80's for whatever reason. Kinda like when my old Emotiva MPS 1 kept shutting down driving my M80's. On paper they say it will, but in reality it wasn't designed to what the paper said. During wide dynamic jumps in dB's they would puss out.

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