After so many years standing for this annoying buzzing sound, I decided to spend some time to find and eradicate the problem at the root.
I hope that sharing my problem and the solution here may help other people.
The first step in solving the problem was to identify it, and find its name.
As long as we can't precisely name a problem, it's hard to solve it, because we can't search about it.
With help of Google, I found the name of my problem: Ground loop.
At first I didn't believe this was my problem, because that house is quite old, and there is no ground at all in the AC outlets.
I was thinking:
I have no ground at all here, how could I have ground loop problems?
Here is a diagram I've made that shows the loop.
(I have let most of the Firewire pins unconnected for clarity)
The loop becomes clearer with the diagram (yellow)!
The strange part is that, if I unplug one of the speakers, the noise stops. But if I unplug the other one, it doesn't.
Now it's obvious why this is the case: unplugging the speaker at the right in the picture doesn't break the loop but unplugging the left does.
We can now also understand why unplugging only one RCA doesn't change anything (all RCA need to be unplugged for the noise to stop).
The solution: breaking the loop
We need to break this loop. Of course, unplugging the RCAs is not an option (we wouldn't hear anything anymore).
The solution I found is to unplug the ground cable between the left speaker and the power strip. To do this, I used following device (3 to 2 pins ground lift adapter):
While it's usually not safe to lift ground in this way, in my case it's not an issue at all since my wall outlets don't have ground at all in the first place.
Following adapter seems to be safer (in case there is a real ground): it keeps the existing grounding but filters the current passing through it.