Congrats to the OP on your new saw! true it may not the fastest saw at a gtg or the new kid on the block but what you have is a proven dependable day in day out 24/7 work horse!...Sort/mod the muffler and retune to get it back up to specs as guys have already said, but seems you know that already ordering the daul port muffler...
I don't think the 660 is an antique just yet?? happy cutting and be safe out there .
Here is a vid from a day or so ago of one of my old stock aussie ms660's - 25in hardnose bar 7pin RS .404 bucking some small aussie hardwood.... It's a little long in the tooth this saw with high hrs on it but still seems to run ok to me, has an old 660 crank in it out of a parts saw as it snapped the crank a few years ago, out of all the 660's we have it's the only one that ever snapped a crank...
Disclaimer this is not a how to use a chainsaw video! do your own thing and be safe peoples.