In Windows controls, set Volume to exactly 100%.

Now run RME Bit Test (see ADI-2/4 Pro SE manual page 101) to check if you found the optimum.

If Bit Test passes your files are played without alteration, can‘t be better than that.

Bit Test is a special function built into ADI-2, very simple to use:

Play the special files for each resolution, supplied by RME, and see the “Bit Test xxx passed”-message on ADI-2’s screen.
No message: system configuration sub-optimum.

Files:
http://www.rme-audio.de/download/bit_test_wavs.zip

Yes, because ASIO bypasses the windows sound subsystem, then the Windows mixer is also out of the loop.

If you do not use an application with ASIO support, then you play through the WDM driver, which is Windows compatible. There, the Windows mixer is active. The WDM drivers are automatically installed and in Windows sound you can choose, e.g., the analog or phone channels to become the Windows default sound device.

If you open DAW in parallel and use the ASIO driver, then it's easy to see, that ASIO bypasses Windows sound.
The volume control of Windows mixer has no effect on the DAW volume but on Windows Sound (YouTube, ..).
Easy to test. I just did that for you.

So … When working with the DAW, use the ASIO driver. While working with the DAW it is recommended anyway to not use other applications or to use the network / browse the internet.

If you want to use the ADI-2 Pro FS for Windows sound well, then simply use the WDM driver, this you can do by opening Windows Sound and setting the ADI-2 Pro Analog Out 1/2 as the Windows default sound device.

You can also use a Music player like MusicBee which supports ASIO (in contrast to foobar2000 without plugin) to listen to music. Additionally the player offers many useful extra features and the look is customizeable.

You can even use ASIO and WDM in parallel, as I mentioned.

Thanks, I understand.

Tried it with MusicBee also, same results. No message pops up on the interface.

How do I activate ASIO in the ADI to use in conjunction with foobar (ASIO output plugin already installed) ?

Indeed, my bad. Now the bit test passed in ASIO mode on foobar.

If you don't mind, a follow-up question :

What output reference level should I choose for optimal playback quality ?

I'm using an electrostatic headphone amp out of the analog xlrs of the ADI, so I got potentially 2 volume knobs, the one from the ADI, the one from the headphone amp, but I'm unsure which reference level I should choose for the best quality and avoiding "strain" on my equipment.

Do you want to use dynamic loudness? If yes, then using auto ref level would also make sense.
Use then a volume range up to a certain max volume level which doesn't overload the inputs of your preamp.

If a 440 Hz sine wave starts to distort, then you know how far you can turn up the volume of the ADI-2 Pro with auto reflevel enabled.

On the electrostatic headphone amp dial-in a comfortably max volume.

Keep it at that level there. Now only use the volume knob of the ADI-2 Pro...

Configure dynamic loudness and set Low Vol Ref to the lowest listening volume. For me, this is -50 dB.
Then  dynamic loudness starts at -30 and will increase bass and treble dynamically over a range of 20 dB and reach the highest Bass / Treble increase at -50 dB.

The max Bass / Treble increase is per default +7 dB and can be customized as well from 1..10 dB in 0.5 dB increments.

Ok thanks, I thought there was a real difference in quality, but that's probably my ears playing tricks on me.

An important reason for me buying my first RME device a few years ago was the excellent documentation. And let me add that it's updated as well.

Of course you can ask, no one here has suggested otherwise, though reading the excellent manual and then come back if any issue is still unresolved is encouraged.

Firefox will use the audio device you have specified in the Windows Audio Settings, just like most other application, as it is not aware of ASIO drivers as such. DAWs, as part of their nature, handle this explicitly. Very few other application types does, though.

For me, at least, as a consumer, the limiting factor is the source material, and for this Windows Sound (or what they currently call it) sounds great to me. I do have Foobar2000 with the ASIO plugin, but to be honest, I can't hear any difference by casual testing, so good enough for me.