What do you think if i lower the gain of the audio interface to zero, That means he won't increase the signal at all?
Although unity gain on an analogue input doesn’t change the level through the preamp circuit, the operating level of the interface will become a factor when translating to dBFS after the ADC stage. The UK standard operating level is 0dBu = -18dBFS. The US standard equates to 0dBU = -24 dBFS (usually written as -20dBFS = +4dBu), and there are other standards in between. Most audio interface manufacturers plump for something in the middle to try and integrate with as much gear as possible without multiple versions being required, so without looking at the specs, you can’t know how much to offset your measurements by. This also applies to the analogue inputs on professional mixing consoles, analogue and digital, where operating level is either fixed or selectable.
If you go via S/PDIF, you circumnavigate all of the analogue stage operating level variables.