Hi Tabula!
Thanks for the full report.
I don’t have problems with noise at normal gains in my 2 mixer desks (Yamaha & Behringer), and most of people don’t have it, but i have experimented this kind of problems with some zynthians, i don’t know why … I tend to think it’s a grounding problem, but it’s difficult to know. Normally the noise vanishes when moving the wires inside or taking care of having good grounding, good contact between case and connectors, etc.
Anyway, i’m working for improving this kind of problems on future kits very soon 
Regards,