In short.
Modifying volume level in the digital stage, can be noticeable at the analog end (speakers).
This is not caused by the actual audio signal, but due to the noise level. The processed original audio signal can be lowered, however the noise remains on the same level. (The SNR will get worse).
Please have a look here, it’s pretty well documented and easy to read/understand.
How to test
If you have a pre-amp or a DAC with analog volume control, you can simply test the impact of both digital and analog volume control.
Set the digital volume control of your media player (volumio) to 100%, hopefully this is 0 dBFS.
Play at a realistic level. Turn down the digital volume (Volumio) and increase the analog (Pre-amp) at the same time trying to keep the level constant.
