Veseli

I would appreciate some opinions from more experienced techs here, thanks in advance.
Have a audible ground sounding noise with gain at 0 recently on left channel of Art DPS II preamp. And when gain is pushed higher than 75% something pops loudly and the hiss starts and meter goes to max though the hiss doesn't seem loud but I guess it oscillates at higher frequency
Seems to me that their V3 system has failed somewhere as this hissing, meter slamming noise starts when V3 switch is engaged from neutral or vocal to guitar position and lasts all the way up to A-gtr position.
I checked and resoldered all the XLR combo jacks on PCB, changed the tube to brand new RCA tube and no progress so far. I suspected the power supply too but I would have expect both channels to behave the same in that case.
I know its a cheap preamp but it has served me well in past 3 or 4 years for room mics mostly as it has lots of gain. I'm the first owner and preamp hasn't been modded apart of tube change.
If any of the symptoms I described here rings a bell and it is worth checking out please leave your opinions. Thx again.