EDITED:
I'm sorry, Gordon, but these are two different pairs of shoes, and the comparison is invalid. You're mixing the terms, and come to wrong conclusions.
Tube sound is something that can be both heard and measured. But we are talking about something else, namely audio data that has already been digitized once and is now available digitally in a long sequence of "zeros" and "ones", simply binary values, nothing more.
You have noticed yourself that the transport of this digital audio data (binary values) to the internal CPU of the ADI-2 Pro/DAC is error-free. So the data (each Bit and Byte) has been transferred via USB, and is now internally available in digital form and unchanged.
If nothing changed in the data (zeros and ones), then there can be no change in the sound.
That should make sense to you, right?!
Jitter is also no longer relevant at this point. Steadyclock has compensated for any jitter so that you have a stable digital connection.
And from the design of the device the final D/A conversion (what restores the analog sound with the AKM chip) is done with the internal Femto Second (FS) clock. This means that any jitter on the digital line (USB) is without any effect at this point for D/A conversion.
The high quality of the ADI-2 Pro/DAC's D/A conversion is the D/A conversion quality of the AKM chip, combined with the precision of the internal FS clock
If you think you hear any differences now, then they are simply psychoacoustic effects.
It's worth checking out.
In order not to be blindly exposed to psychoacoustic effects, two things are worthwhile:
a) to know that they exist, how the mechanisms work and to act accordingly according to the recommendations regarding tests / methodology, etc.
b) to know how digital audio transmission works plus the RME specials in the design of the devices. Then you can perform logical conclusions that some observations maybe are not true and a matter of psychoacoustic.
Without these things you don't have the right orientation, and unfortunately you are simply at the mercy of all this HiFi Snake Oil nonsense.
From a setup point of view it would also be generally recommended to remove things from the signal path that are simply useless and just another potential source of error, or that might even become damaged over time and then perhaps introduce interference into the system that might even be unnoticed at first and then later can only be found out with increased troubleshooting effort.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13