In my opinion, nothing to worry about. Did you ever make a magnet out of a nail? You wipe a magnet along the nail a bunch of times and it magnetizes the nail. Well you can de-magnetize a magnet too. But it takes repetitive changes in the magnetic field. Putting two magnets next to each other does not cause repetitive changes, or oscillations, in the magnetic field. So no big deal. In fact, magnets are shipped in stacks together. But a speaker has a varying magnetic field in the coil when music is played through it. That oscillating field could slowly de-mag a magnet. I doubt it could do it much to your pickup at those distances, though. Most speakers, electric motors, and microphones are built with one coil and one magnet. The magnet doesn't weaken with use. So, while theoretically your speaker might de-mag a pickup very close to it, I really doubt it is a real world issue.