I have the 8030C. The answer is two fold --- there's an analog potentiometer behind the 8030 to tune your speaker's sensitivity.
There's also a DAC output level to set its gain.  You can turn the two together.

If you want  to do the mathematics, here's the easy way ---
turn the knob at the back of 8030C to +1db, and set your ADI-2 DAC's output level to +1db.
Now, if you play full volume content at 0dbr volume, and sit 1m away from your speaker, you will hear a sound that is 100dbSPL loud.
That's what the sensitivity spec label means.

All others volumes can be calculated relative to this.

For instance, if you turn the output level of the DAC to +19dbu from +1dbu, which is 18db louder, then you'll hear 118dbSPL at full volume.
Another example --- keep the 1dbu level, and turn the knob at the back of your 8030C from +1dbu to +6dbu, and you'll get 5db quieter, the sound level is at 95db.

So what you really want to do depends solely what listening levels you want it to be. You can turn the knob to +6dbu, and the dac level to +1dbu, and digital volume knob to -infinity, that's the lowest sound you'll get, and you slightly increase your dac's digital volume from -infinity slowly to 0dbr. and stop when it's loud enough.

if this is too quite for you, turn the 8030C knob slowly from +6dbu to -6dbu. The sound will get louder and louder. Stop when you are satisfied with the loudness.

If it's still too quite, keep the knob at -6dbu, and turn your adi-2 dac's level from +1 to +7, +13 and +19dbu. The sound should get louder and louder. Stop when you are satisfied.