Not sure if this is the answer, but every speaker is designed differently, has different efficiency ratings, and different ohm ratings. The C-7's have a normal impedance rading of 8 ohms, so their resistance is different to the 4ohm m80's. So that would make sense they would take a little more juice. This is why many receivers have issues with 4ohm designed speakers, nothing new here.

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