Yes 40hz is the minimum crossover given the M80s extension to 34hz at the -3db end. It depends how well you blend it with the sub in the context of your room and listening postion. If you have the flexibility I would also try 50hz and 60hz and listen to what sounds better at various volumes. A key benefit of crossing higher is that you free up amplifier bandwidth which can mean less distortion. However, a more powerful amp usually means you can cross much lower with confidence since preserving amp efficiency is less important.

John