That's easy. For this your DAC / Preamp needs to have a digital volume level display, and a step attenuator (the Zodiac Gold does). You can simply ask the maufacurer what volume level corresponds to "unity gain" (i.e. 0db attenuation).
You connect the HT mains to the analog input, set the DAC/Pre to this unity gain level and you're in business. In fact most HT bypasses in preamps work like this - they don't bypass the active circuit, but fix the level at unity gain.
If the maufacturer does not know you can find the unity gain level yourself using an SPL meter, but you don't even really have to. The manual will tell you the gain of the preamp and the Db increments. So if your preamp has 10db gain and 0.5 steps increments and has a scale of 0-140, you just set the level to 120.
Better still, you don't even have to be at precise unity gain, because after deciding what level to fix the DAC/preamp on, you next trim your channels on the MCH sourse (typically a SSP) using the SLP meter. This which would synch up all the chanels even if your DAC/Preamp is not at exact unity gain.