A picture really helps.
That is a horrible and crude way to mount binding posts. Obviously the back of the cabinet has become conductive possibly through humidity.
I would never design a speaker connection like that.
You need to mod both speakers before you blow up your NAD.
You need to get some plastic, and cut it square. Then you need to cut a square hole in the back of each speaker, that is a bit smaller than the plastic. Mount the binding posts to the plastic and then screw the plastic to the enclosure.
A better alternative is to replace those binding posts with a terminal cup.
This is your best solution.
This is Triad's fault, for crude and nasty design. Speaker companies just give you the right pip.
If you want good speakers, build them yourself!