I didn't think you'd hear much difference with the M80s and a sub when bi-amping because the M80s are more difficult to drive below 50 Hz compared to the rest of the spectrum. On second look at the impedance and phase response though, they continue to have "low" impedance and leading phase angle right about to the lower mid-range. So if you are driving them hard, you may hear a difference when bi-amping. I'd be curious to know what you hear.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated