Well I believe it does not support the speed of 1666mhz as stated on the Asus website:
" 2x DIMM, Max. 16GB, DDR3 1333/1066 Hz Non-ECC, Un-buffered Memory
Dual Channel Memory Architecture
Supports Intel® Extreme Memory Profile (XMP)
* The maximum 16GB memory capacity can be supported with 8GB or above DIMMs. ASUS will update the memory QVL once the DIMMs are available in the market.
* Refer to www.asus.com or user manual for the Memory QVL (Qualified Vendors Lists).
* Due to OS limitation, when installing total memory of 4GB capacity or more, Windows® 32-bit operation system may only recognize less than 3GB. Install a 64-bit Windows® OS when you want to install 4GB or more memory on the motherboard."
That being said, I believe the motherboard will downclock the ram to 1333mhz. Anyways you sure it isn't 1600mhz?
Edit: Darn it, beaten by 10 seconds