@ BLunTSmoKeR, I would ask them to take the mobo back and pay a restocking fee if necessary, and send you a new one. That is why I will only buy some things like motherboads from places like newegg. If it does not work, you rma it, none of this in house "testing". Only one I trust testing would be manufacturer.

I would have 1st tried different RAM and slots (which you did). Second tried different cpu, though that would be very low yield and only since easy and if I had one and then I would have rma'ed the mobo, you have to try another mobo, that is next logical troubleshooting option. Anything else in inefficient. I would just not take no for an answer.

One other thing, I assume you have tried using load optimal defaults, and then ran that using prime WITHOUT changing your memory to 1600, ie leave it at what boots up. Changing RAM means OCing uncore and OCing ram (not according to your ram specs but Ocing according to cpu/mobo) because if works at true stock, ie optimal defaults then might be a setting. Though the passing sometimes failing others...again I would try another mobo.