All the digital inputs go through a digital switch and then into DAC chip. The DAC chip does volume control as well as converting it to analog.
The output of the DAC chip goes from 0 to 2V max, and then to the internal power amp.
So when you turn the volume knob on IDA-8 you are changing the volume inside the DAC.
But for higher end DAC such as DAC-10, the volume control affects TWO places: the DAC as well as the resistor ladder preamp after the DAC.
If the input is digital, then the preamp doesn't change the volume (it just switch it through a 0 db gain), the volume is best controlled inside the DAC.
But if the input is analog (remember, in this case there is no A2D conversion for DAC-10), then volume gets changed by the preamp after the DAC.
It is kind of complicated for non tech people, there is an FAQ under the DAC section about this, I am not sure if that helps.
All this trouble just for one analog input
I know for some people that one input is the most important.