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

Make sure the ground connection of your cable TV is clean and solid. Next, I would check if you have any halogen lamp dimmers and/or ceiling fan variable speed controls. These devices throw a huge amount of noise into AC power. If that fails, the repair shop may be the only option left, after you have isolated the problem component by deduction.
Good luck, I know this must be annoying.

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.

Is all your gear on one plug or circuit, meaning components on one circuit and amps on a seperate. If so, you may have a ground potential issue.(use an extention cord and test by plugging all devices and amps in same circuit) I am presently experinecing the same thing and found cheater worked on my pre-amp and amps, I basically lifted the ground..

Confused about the x-over issue, see if you can borrow the same preamp or another and test.

Matt

Many thanks to all the participants of this forum. I am trying to use all your help and have some answers and more questions to ask.

Ktsteamer:

It is an interesting possiblity. I indeed have the room's lighting on a dimmer switch in a different room, which is exactly above the listening room. In my experiments, I made sure that it is swithched off, but I wonder if, even so, it may still creat a buzz???

Mzn50: I plug them all into one power surge protector and
line conditioner. I tried the cheater and checked the polarity too. I also connected the chassis of the preamp to other various components. The crossover I mentioned is in the separate power amp for the sub, that is, not in the preamp in question. So, I can run the full range signal from the preout in the preamp, and then can cut it off below 80hz, by engaging the sub amp crossover. This kills the buzz in the subwoofer speaker. Hence, it is not a 60hz ground loop hum?

Thanks.

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.