Unclear description:

> Aqvox PH2 mk2

Is that the Aqvox Phono 2 CI MkII? Is see it has Earth connection and a Ground Lift switch.

>  The DAW and the RME are connected with a two pole (no ground) plugs through the AC/DC adapter

And your monitor? Also is that a laptop used as DAW? Does the hum go away when used on battery?

If I remove the preamp (i.e. disconnect the Aqvox from the A/D inputs of the RME) no problem: I can listen digital files w/o noise at all. The noise appears just when I use the A/D inputs with USB connected

I do not use phone output at all

Thanks much

Try a headphone connected to the ADI just for testing. The ground loop likely happens between the TV (via HDMI shielding) and the PC, both of which are connected via 3-prong power-plugs (earth/safety line).

When you disconnect the TV, how did you listen to the noise? If you connected speakers that use a 3-prong power-plug then you are back to a ground-loop. Sometimes it can help to switch wall sockets or to use a multi-socket if multiple earth connections are present. What happens is that the travel distance for the different noise signals are shifted slightly so that they may either amplify or attenuate due to phase differences. I can reproduce this by using different sockets for my computer display when I create a loop in combination with powered speakers and PC.

If you are using balanced cables for your speakers then you can try to break/disconnect shielding on one side of the cable. On XLR cables this is pin 1.

The amp itself can also be ground noise sensitive, which I guess is the case, since there is no noise on the headphones when connected to usb and the rest. So the noise comes via the ground from the laptop to the amp, but is not from the RME. As Timur said, removing the ground from the amp or disconnecting ground in the xlr cable might help.