For the M80's I think you should set those to large (fronts). The center (VP150) should be set to small. Crossover is the point when your speakers stop receiving the signal which is then picked up by the sub. So if your crossover is set to 80Hz then all sounds above that frequency is being sent to the fronts and possibly other speakers, but sounds below 80Hz will be sent to the subwoofer. I'm sure other members can explain it better than me.
Also, try using the THX Optimizer that is included with most (if not all) Star Wars movies. They have optimizations for Audio as well as Video. I played around with my Audio and Video settings with my new Oppo player. These tests might reveal if you anyone is out of place (ie. speaker connected to different terminals or if they are out of phase).
One other mistake I made with my M60s is that I removed the metal bracket connecting the 2 sets of binding posts. Just make sure to leave that metal strip in place or else half of your speakers will be receiving the signal from your amp.
