This is likely caused by volume level from your player that can reduce/increase volume digitally.

Reginald,

the playback volume should not normally change. Perhaps the player uses replay gain tags that may already be embedded in the header of your audio files, but may or may not be used depending on the player or its settings. Or you may have different volume settings in different player.

With Windows, it could also be that they used different drivers.  An ASIO driver bypasses the whole Windows sound system. But maybe you used another Windows audio driver and possibly the Windows kernel mixer was in the signal path or Windows sound extensions were enabled.

So please use the bit test, see also the manual.
Then you can play the test files in WAV format in the player and then observe if the ADI-2 DAC or Pro confirms in the display that the test was successful. Which means, that the signal path from the audio player up to the DAC (shortly before the D/A converter) is lossless.

You just have to check the details, and the manual is your friend.

Kai, what you describe here is in my opinion an entirely different topic, or where do you see a parallel to the original question of Reginald?
This takes place at the end of the signal chain around digital volume control, with two different reference levels at the analog output.
Maybe you should have opened another thread for this so that these two different topics do not mix in this thread.

Reginald's issue / question is purely at the digital level, where it shouldn't make any difference which hardware player he connects via USB, provided the starting conditions are the same: same audio material, same digital output volume at the player, replay gain tags turned off, no EQ'ing or add-ons that manipulate the sound or add any kind of room emulation, etc.).
And USB is of course sonically neutral, pure lossless transport of digital audio data (zeroes and ones).

All good Kai. In the first place, I just wanted to understand exactly what you mean. I wasn't certain whether you see a connection there, or whether you just had another topic going through your head that concerns you in this context.

Currently, I still tend to think that the two things “should” have nothing to do with each other. Because of lossless audio transmission in the digital domain vs. different control of analog output stages behind the D/A converter at different ref levels.

What you describe, I would perhaps put under the rubric that the analog components might sound slightly different, when using different ref levels … Maybe something like a sweet spot of sound that matches better to your listening habits.

But I assume the differences you hear are so small that you might not be entirely sure yourself, are you? A blind test would probably be really interesting here, whether you still perceive the differences blind and then get them clearly assigned whether its A or B.

Keep it here, I made this comment only under the assumption that these are different topics, but ofc I could be wrong.

Hello all, I hope you do not mind me scratching my head here...as said too technical is a wasted with me, I only made the observation, and wondered why a signal that is purely digital is different in sound level when the exact same files are  played from different players (FIIO M15 and 2021 Ipad mini) through USB...
Again, using the audio level settings on as well the FIIO and the iPad does not change a thing. ( When I connect my smartphone ,Huawei Pro40, through USB C to my ADI then there I can change the volume setting on the phone which is audible when playing through the ADI)

I followed Ramses's advise and ran a BIT test on both players, and they both passed.

So is it the case that as well as the FIIO M15 and the iPad Mini both have there set audio levels going out through USB?
If that is the case my question is answered.

Still leaves me scratching my head why (wonder now if this is subconsious) the FIIO sounds "cleaner" than the iPad
(my headphone is a  Hifiman HE R10P's used in LO-power)
So I do understand where Kai's confusion is coming from as well..

If it wasn't for the WIFI connection problems of the FIIO M15, I would surely stick with that.
The iPad mini is much more stable in that sense. 
Of course the problem only occurs when using a streaming service (TIDAL)
For local files there is no problem at all using the FIIO..

So to finalise does each source, in my case FIIO and iPad, have their own set audio levels that are sent through USB?

Thank you all for the replies and thoughts it is really a big help.

Cheers
Reginald