To be certain, you'll have to use the Right and Left Front channel input connections of the 7.1 analog multichannel inputs.
That will, of course, prevent bass management and other features that are provided by the 8801's processor. You could use Y connections to connect your DAC to both the 7.1 inputs and one of the stereo inputs. That'd let you compare how the different options sound.
It is possible, but not known for certain, that selecting Direct will avoid digitization of the stereo inputs. Nobody has reported seeing the circuit diagrams so we'd know for sure.
How would one find out ?
Volume control by the 8801 does not require the 8801 to digitize the input signal. I.e. volume control works fine when using the 7.1 multichannel analog audio input.
Bass management does require (re)digitization. I.e. bass management does not work when using the 7.1 multichannel analog audio input. It works fine for any of the stereo inputs if Direct or Pure Direct is not enabled. They're all (re)digitized.