I see my misunderstanding now: the only possible highest frequencies which can be produced by the AD9523-1 with the 122.88MHz VCXO are 983.04MHz and 768MHz, which I found by selecting the settings for the AD9523-1 within the ADIsimCLK tool. For anyone who has a similar question, I would recommend checking ADIsimCLK | RF Tools | Analog Devices.
For future reference, how do I change the Doubler value to 1, and the prescaler counter values A and B to B=6 and A=1?