Recently acquired a pair of NC400 mono amps, and nothing obviously stands out as wrong with them, but they definitely weren't assembled by a professional assembler, and when they're powered on the NC400 modules have strips of red LEDs all lit up, which... anecdotally tends not to align with STATUS_OK in my varied life experiences.
I couldn't find anything about these LEDs in the Hypex datasheet.
Attached are some images of the wiring on the inside of each mono amp. It seems like it's wired correctly, though the "Mute" switch inline of one of the connections seems odd.