Well, since your driving m3's, this is puzzling, they are not a low ohm speaker so that must not be it...
Receivers generally go into protect mode if:
1) they get hot because you don't have adequate ventilation "around them".
2) if you have even "one" tiny wire strand touching the other terminal on the receiver, speaker, or if your wire insulation is bad somewhere alone the run.
3) if something mechanical is wrong with the receiver.
Also, when your ready to get another receiver, I would not limit your choice to Marantz. In my opinion Denons offer the best features for the money.
