My NAD T747 keeps shutting down in Protection Mode at 6db when it used to go much louder. I cleaned the unit. Still nothing.
Modern AV receivers such as your NAD have protection circuits to prevent damage when there is a short on the speaker terminals. Look carefully at the speaker connections on the back of the AV receiver- for example, the speaker cable for front left speakers labelled + and - : there should not be any stray strands of wire that connect from + to - ; this would be a short circuit. Check each of the connections on the AV receiver as well as on the back of each of your Speakers themselves. Also ensure all connections are snug (do not use pliers). If this does not solve your problem, you may need to take your AV receiver on for a service checkup. Good luck !