Not sure it would be harmful, as the m80's and 180 can play very low frequencies. However, a subwoofer with a 12" or bigger woofer can move more air in those lower frequencies. When you set the crossover to say 80hz, frequencies below that point are redirected to the subwoofer. Also, some think they are just cutoff, when in fact there is a slope or blending handoff.

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