i would say that the receiver is not on its last leg...
i would say you are trying to get more power out of the receiver than it was designed for... with the M80's you have 2 4ohm speakers... now your running 3 4ohm speakers... that will create a LOT more demand on the receiver, the power supply that was used in that receiver probably can not handle the load of 3 4ohm speakers. 2 4ohm speakers + the rest of your speakers is probably right at the max of what it can handle, you were probably operating right at your equipments design limit the whole time, and didn't notice it... when you threw the 180 into the mix, it was just enough to push it past what it was capable of.
i would say, get a nice 2 channel amp, put the M80's on the 2 channel, and then use the receiver for the rest of your channels and you'll be fine. unless you want to buy another receiver...
You were on the right track, with making them use the 4 ohm speakers at the store, this time make them connect 3 4ohm speakers to whatever receiver your looking at buying.
