NAD C375BEE protection circuit activating.

I recently purchased a NAD C375BEE amplifier from an AUDIGON member and it ran fine for two weeks. Last evening the protection circuit activated after playing for a few minutes, I checked the speaker connections included those for my REL subwoofer and all were tight and not shorting yet it activated again after a few minutes of playing. I know the basic routine for trouble shooting but would like some advice and I would also like to know if activating the protection circuit several times while troubleshooting might damage the amplifier. BTW my previous amplifier was an Anthem Integrated 2 and there were no issues.
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Thank you,