If a cancelling magnet is going to completely cancel the magnetic filed from the magnet used in the drivers in your speaker it would have to have exactly the same magnetic characteristics as the magnet used in the driver. This will never happen.
What does this mean in practice? The magnetic field will be mostly cancelled but will still exist, though at a lower intensity. Electrons do not weigh very much, 9.10938188 * 10^-31Kg, and on top of that do not need to be deflected very much in a CRT to fail to hit the phosphor dot they were intended for. Hence even a weak magnetic field will deflect the path of the electron enough to cause interference.
Thin tin foil will probably have zero appreciabe effect on the magentic field from your speakers. There are companies out there that will sell you magnetic shielding foils, but these tend to be thicker and made from other metals. The price is also sigificantly higher.
The next point to make is that putting larger speakers that come as part of a stereo on either side of your monitor and sitting half a meter in front of them is actually a very poor listening position (Please note that the 480W rating refers to their peak handling capacity and not to the constant load capabilities). They will not be designed to form a decent stereo image at this distance and spacing. the frequency response curve for such a listening position is also likely to be completely distorted. (This will be mostly due to the table they are sitting on acting as a massive soundboard adding all sorts of resonance and splashback issues)
Hence you will get a better sound out of them by positioning them correctly which would mean moving them away from your monitor. This would also have the effect of removing the interferance problem.
Is it just me, or does something smell suspicious about all this?