Eliminates the hum after 10 months

I got the boxes at a re-opening of an electric market extremely cheap and had to deal immediately with mains hum (?). I've read through some forums and follow the advice. Whether plug turn, in another outlet, ferrite core, mantle flow filter. Nix has availed. When it turned louder the hum was less noticeable, otherwise very clearly. First half turn down only bass. After about 3-4 months, muttered suddenly even if the bass was completely out!
Because it helped only a an object (in this case chewing gum can) be tucked under the subwoofer, so that the speaker cone could not swing.

WHY DO I HAVE JUST RETURNED PART NOT interested?
I did not want to invest any more money after I bought several headphones already for 200 and the satellite made for the small coins which I had invested for the boxes, a solid impression. Also, I'm not the bass fan, but no bass of course, is a pity. To be precise: I was too lazy :)

Now how solved the problem?
After 6 months, I have further rearranged, forgetting to pinch a little below the subwoofer again. I had accidentally the Bass control turned slightly. I felt like the pleasant bass without the constant background hum. Even if one is turning up fully perceive no hum. Now I have been a week simply a great listening pleasure. As I said, the satellite do their job well and now they are finally readily supported by the subwoofer.
So my theory: In about 8-10 hours of music a day (mostly Internet radio) and the object under the subwoofer, the membrane was somehow "burnt".

I know that for more than 90% of people with this problem that will not be helpful here as the hum can have many reasons. But this "solution" I have not seen before, and if it would help just 2 people, it would have been worth it. For 30 (yes much I paid) absolute decent boxing. Up to 60 I do not know any better 2.1 PC speakers. In addition, there are of course better alternatives. The prerequisite is that the device is running without errors. Logitech formerly stood certainly more for quality than today. The devices themselves are usually processed properly, but the rate of the small or large Problemchen has become higher for me.
The bass control is simply brilliant. Bass fetishists among friends can turn up the bass while I can enjoy alone just relatively neutral sound with bass smoothen.

Supplement June 30, 2013: The humming is coming back, but the problem has been located.
I again turned on the power after a weekend and the subwoofer immediately began again to hum. The catch is not stuck in the laptop, as it always buzzing, I thought to myself. It hummed on. I plugged my laptop into the AC adapter and the hum was gone. This should be just another mosaic pieces to solve the problem. For I know that the hum 10 months ago was also with power to the laptop there.