Chris, welcome. If the "bi-amping" that you mention is simply hooking up the back surround channels to the mains, forget it. This isn't biamping and is still limited by the available power from the one power supply section in the receiver. Available maximum power isn't doubled, as some imagine, or increased in any amount.
As to whether this available power would be enough for your use with M80s, this would depend on how hard and for how long you work it. In most cases it should be sufficient for the M80s, which are slightly more sensitive than average and require less power than many other speakers, whether rated at 8 ohms or 4 ohms. Factors such as your listening distance, sound level you use and dynamic range of the material you play are key factors.
