If it's that simple, what is the advantage of XMP? The mobo automatically selected settings that work. For some reason, it defaulted to 1.585 volts. I reduced it to 1.5 and ran memtest for 10 hours with no errors.
Often the motherboard will pick the first one up by default, but not always. Sometimes a set of RAM might have 2 or 3 XMPs, with the 2nd and 3rd having more aggressive speeds, timings and/or voltages. The RAM I bought specifically said that the most extreme profile was not guaranteed to work, and sure enough it was like 1600mhz CL7 1.8V (Way too high).