If I get you right:
you want to connect your ADI-2 DAC directly to a power amp, with no preamp.
This is possible and works fine.

Yes you can! When you have a pre amplifier : connect the Adi 2 dac with your computer (i have an usb connection from my mac mini to the Adi 2 dac).
Connect the Adi 2 dac 2 rca outputs (left and right) with 2 rca inputs (left and right) on your pre-amplifier.
After that you can use the Volume of your Pre.
By the way, with apps as Audirvana or Roon etc. you have an extra digital volume possibility and
can mix volume from both the Pre app. Or use the Volume on the Adi2 dac in combination with your Pre.
Michael

There is no (analog) pre section in ADI-2 DAC.

• Disable all digital DSP functions that you don‘t want to use:
Equalizer, Bass/Treble, Loudness, Phase etc.
• Set Auto Reference Level to “off”.
• Set Volume to 0 dB.
• Select the Reference Level that fits best to your other sources on your preamp, like CD player, Tuner, Vinyl disc player etc.
• Choose the DA filter that sounds best to you (LD Sharp is default).

Now ADI-2 DAC behaves like a normal DAC without DSP extras.

Some of the settings above are factory default, but it doesn’t hurt to go through the list.

one last question, is it possible to use the two RCA and xlr outputs simultaneously?  I would do it to drive two amps