First of all you want to leave the Yammy on the 8ohm setting, if there is a switch on the back for 6 ohm, don't change it. In regards to the M80's, there have been some in the past that have had issues with certain Yammy models going into protection mode after playing for sometime because they get to hot driving 4 ohm speakers.
Not sure about the 2600, you could call Yamaha. The Denon's and HK's have no problem driving m80's because of their overbuilt interior guts.
I can tell you there is nothing wrong with M60's man, they play plenty loud, and my room is almost 900 sq ft. The m60's and m80's have a very similar sound, the m80's just play to more insane volume levels. You may not need the m80's. How big is your room and what are your plans for the speakers, music or HT, and how loud?