I'm waiting for feedback from the manufacturer. Unfortunately I'm in Europe so LTA may not be the best option but thanks for the suggestion, always good to have options.
I made this little clip:
it was playing well for an 1h and after I started noticing this hum once again. I turned off the music when I recorded the clip, actually even without any signal the hum is there. I even test it without any tube and the hum is there. As soon as I turn off the dac, it goes away, turning up and down to volume nob also increases / decreases the audible noise.
:(