Thanks
@Darkglass and
@sunbeast for your quick replies!
It is clearly the cab sim. As soon as I turn it off, the hiss disappears. It happens exactly as you describe it. The level of the bass signal goes up, the hiss remains. The thing is that the hiss is still quite perceptible when I'm not playing. Of course it disappears while playing, but when there should be silence, you can still hear it through the PA, so I ended up turning it off, which kinda defeats the purpose of the cab sim.
Yes, I've tighten the nut and it is fixed now. My point was rather, that this is a NEW, 360 EUR+ pedal. That nut shouldn't be getting lose after 4 or 5 rehearsal sessions. I notice that if it gets lose and the switch rotates around its axis more than say 60 or 70 degrees, it starts to work faulty, turning itself on and off unreliably. It is not really easy to tighten either, so if this happens while I'm playing live, I'm in quite a pickle as this nut is not really easy to tighten while avoiding axial rotation of the switch, and frankly, going back to my point, I don't want to be doing this on a pedal of such a high price point. Don't get me wrong, I love the sound of it, it is really versatile and a great addition to my sound, but that makes these problems quite the bummer.
Cheers,
LEo