You need an external attenuator, or the DAC's noise floor will ruin your measurements with such high gain values.

Even an analog volume control won't be a full solution, as it is not at the output of the unit, so there is a similar noise floor. A passive voltage divider is not 'enough', but the best solution!

Example for 60 dB Pad

Balanced: 3k3 - 6R6 - 3K3 (6R6 made by 3R3+3R3 if not available)

Unbalanced: 3k3 - 3R3

The ADI output should be set to +19 dBu. For THD measurements set the output level of k1 to -3 dBFS, this gives the best THD and THD+N values.

For best resolution of low level signals the input should be set to +4 dBu. The SNR is 119 dB then, so compared to +19 dBu with 121 dB you can look much deeper into the noise floor. Of course only as long as you don't overload the input this way.

Note that a preamp's noise is usually measured with 150 or 200 Ohm plug at the input. The above Pad brings the preamp to its max performance, equalling a shorted input.

Correct.

If it doesn't need to be pro, no box, is also a solution, just build it in the cable. If you get radio interference (probably not) you will hear it. Then you can pack it in aluminium foil and if needed connect that to ground.