Are you populating all 4 memory slots for that 16GB? if so you wont get 1866 to run, or its unlikely. The CPU mem controller does not support 4 slots at 1866Mhz. 2 slots you can go 2Ghz + on the memory but once you use all four you are 1600Mhz for standard clocks. I run 32GB on a Sabertooth and manually set my speeds to just over 1800mhz, but it will not boot at 1866Mhz. The speed difference is unnoticeable if it makes you feel better.
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