Well, the parts I ordered for the MS 660 and MS 461 (pistons for both, etc) finally came in today, took almost 2 weeks!
Then the owner of these saws (a local tree guy) calls me and tells me that he is dropping off another MS 660 in the morning. seems that the local Stihl shop charged him $125 to rebuild the carb, and when it ran like crap afterwards, they just told him that "now it needs a new carb". He rented a crane and everything, put the big bar on the saw, and it just bogged in the cut.
I told him since the other 660 is currently apart, I'll just put that carb on it to see if it is the problem, but I may vac test it first.