Do you have your M80's set to large? If so set the speakers to small in the receiver bass management settings with 60hz or 80hz crossover point. You are asking those drivers in the M80's to do far too much and the fluttering is a sign of distress or possibly even permanent damage.
Get a good subwoofer. In fact...
Iron Man 2 is a bass heavy movie that is mixed quite "hot" on a lot of scenes. It will even make a lot of subwoofers which are specifically designed for this application of producing low frequencies to make unwanted sounds at reference levels - "flap", "fart", "chuff", "rattle", "flutter" etc.
