Many thanks for your detailed answer, I'm not that expert and trying to learning as much as I can from this forum.I think the routing solution you use now is the most optimal. If you'd use the D70 as preamp you'd lose the balanced connection to the A70.
The difference in using the D70 vs the A70 as a preamp is that the D70 has a digital volume control, and the A70 has (a very good) analog volume control.
The measured performance of the digital volume control in the D70 wil be rivalling or outperforming any SOTA analog volume control, including that of the A70. That is for (channel) level-matching, crosstalk and SNR. Will you be able to hear that? Unlikely.
Of course I wouldn't give up to XLR connection to A70, I'd swap them from monitor to amplifier every time I need to use headphone (of course this wouldn't be the better solution)
One strange thing that I noticed is that If I directly connect the Eris Pro Sub 10 to the D70 by XLR cables I have no crackling/distortion on the monitor with the monitor gain to 0 dB.
Once I use the full chaing (D70 -> A70 -> Eris Pro Sub 10 -> Eris Studio 5) sometimes I ear crackling sound from tweeter, for examples when I'm watching youtube videos or listening to piano song (FLAC 86 KHz). I have to set the gain to -5dB on the Eris Studio to reduce this crackling sound.
If I use RCA unbalanced connection from A70 to Eris Sub, there is no crackling/distortion from the Eris Studio Monitor.
Of course using unbalanced connection brings a lot of noise and disturbance in my particular case, so is not a possible way for me, it was just an experiment.
Any idea on the reasons?