What happens if you use the right memory sticks? It is possible that this behavior is expected since you are running the memory out of the specs. The memory specification for this processor are stated here;
Having this configuration (32 gigs of new memory 2666 MHz) can go two ways; it can either work or it simply won't work and from your description I would say that is not working since you already replace the motherboard and the issue persists.
Since this CPU have 4 channel memory, if you just put DIMM on 4th slot and it still beep. It is CPU problem.