Your frequency choice is a little dangerous. As you can see below, many harmonics are falling very close to your fundamental:
sampling rate: 2000MHz
fundamental frequency: 666MHz
2nd harm: 668MHz
3rd harm: 2MHz
4th harm: 664MHz
5th harm: 670MHz
6th harm: 4MHz
7th harm: 662MHz
8th harm: 672MHz
9th harm: 6MHz
10th harm: 660MHz
11th harm: 674MHz
Moreover, are you actually using 666015.625Hz or exactly 666MHz? In the first case, you would be using coherent sampling (the DAC clock and the source clock must be locked) while in the second case you would have your signal energy spread over several bins hiding the higher order harmonics.