Ok, thanks for the answer.
I did several attempts, trying to configure through the program via USB. I managed to get a sound more than usable on some effects, but not on others. For example, the background sound in effect like FreeVerb is much less than the noise under the Jot Reverb.
Regarding signal levels, I’m using a custom configuration with about output on -11, that seems to reduce background noise (and +10.5 on input, but I hear no difference even if I put it to +0).
Using the USB powe, instead of the 300mA power makes a little more noise reduction, but not a meaningful one.
Unfortunately, the amplifier does not have any ground lift input / output, and I confirm that the background sound is present, whether I connect it to the same power socket or connect the amplifier and the pedal to two separate power sockets.
So, I managed to get a good sound, but only with some effects, but I wasn’t able to completely elimiate the background noise, but only to attenuate it.
(Note: the Owl noise is still higher than the background noise of my whole pedal chain without the Owl.)
Tell me if I you need any more information.