The evaluation board uses AD1938 converters. You can see in the AD1938 datasheet that the full-scale differential ADC input is 1.9 Vrms. However, since the evaluation board uses line inputs that are single-ended, the input filtering is actually single-ended, and the filter circuit artificially creates the differential signal.
The single-ended range on this board is actually ~2.8 Vp-p. I would suggest dropping the test input signal down a bit below 2.8 Vp-p and then inching it up until you read about -1 dBFS peak in the DSP core.
You can easily monitor the input level in the DSP using these methods: http://ez.analog.com/message/7669#7669
I would suggest using the peak method. You don’t want to be right at 0 dBFS because harmonic distortion will be a bit high. I would step it down to -1 dBFS for the best balance of noise and distortion.