Hi team04,
I believe that it is convenient to use the profile registers in creating QAM signals. For 16 QAM, it is preferred to use the direct conversion mode using function pin values of 0101b or 0100b. Using this, your amplitude and phase offset can have 8-bit resolution and you can dynamically change it by just accessing the parallel pins. This is illustrated on page 28 and 29 of the datasheet.
I hope this feature can help you out in your project.
Thanks
Louijie