Did anyone manage to ``repare'' a buzz, ever?

It might not be your gear, but the room itself. Sometimes, if there are tracking lights present, or if any of the room's lighting is on a dimmer switch, that might be your problem.
I have that problem now - the lighting in my room is dimmer-switched, and it is wired to all the electrical sockets in my room (all my room's electricity is on one fuse). My preamp is significantly more buzz-y when the dimmer is set higher on the switch. Try taking your preamp to another room with a separate fuse/socket and plug it in. If you don't hear any more buzzing, it might be the lighting.

I'm not an electrician, but I know dimmer switches emit heavy ac waves, even when turned off. So your gear might buzz even with the lights off. If it doesn't buzz hooked up to another socket elsewhere (without a dimmer attached to that fuse), then it's the dimmer switch. Or, if the buzz gets louder when the dimmer switch is set higher, that's a good way to find out too. Also, if the lighting is track, there will be transformers present somewhere - maybe in between the wood beams between floor levels. They also emit ac waves. If you find the dimmer/track lighting is the reason, you can replace the switch will a standard on/off switch, and that might reduce it a bit. If it's the tracking light/transformer, then short of gutting your ceilings and installing new lights, I guess you'll just have to deal with it (I have to). Make sure to throw the fuse switch before installing a new light switch. Hope this helps.