From your description it sounds like you lost the ground connection through your interconnect cable, leaving just the "hot" connections on both channels. This will cause most amplifiers to oscillate, often resulting in damage to the speakers, or amp, or both. It would be nice to think that the internal fuses had protected your NAD amplifier, but the fact that some parts overheated suggests otherwise. If you're feeling adventurous, you can check for blown fuses and replace any you find, and try it again. Otherwise, a visit to your local audio hospital is called for.
Note that this problem may occur again with the same front-end gear, regardless of what amp you are using. Losing both ground connections is rare with separate interconnect cables, but becomes more of a problem with Y cables - especially with mini-plug connectors. Check your cable carefully and replace it if you think it might be bad.
Note that this problem may occur again with the same front-end gear, regardless of what amp you are using. Losing both ground connections is rare with separate interconnect cables, but becomes more of a problem with Y cables - especially with mini-plug connectors. Check your cable carefully and replace it if you think it might be bad.