will a 755 stay cool and on running the mmg's they are 4ohm and only 85db sen and i listen at high vol alot

The 755 can safely drive 4 ohm loads. I wouldn't expect the 755 to have a problem driving Magnepans. There are some people driving MartinLogan speakers with Outlaw amps (a similar load to your MMG's). There may even be somebody around here who has used them and can offer first-hand feedback.

I used my mmgs, CC2 and top end of my 20r driven from the 755. Very nice, plenty of power (had to bi-amp the 20r though)I did have better results with the Threshold 400a, but that is 10x the price per watt (and it eventually blew itself up, am in the process of repairing it)