Contacted Gordon at IQAudio (amazing service and product) and he told me to set the software volume control. The ALSA volume level needs to be set to 0db (86% on linux prior to very latest, 100% on very latest) via the alsamixer. I used the documentations provided on his site and now everything is fine, software volume with a range from 0-100%.
The idea of configurable volume range is very useful because many sound cards (USB but also I2S) don’t top at 0dB but at +3…+30 dB, therefore risking clipping. It is important to limit them.
Software volume is possible but not optimal because it reduces the sound quality on 16 bit DACs.