This explains magnets and color tv screens. http://www.howeverythingworks.org/television.html
Can the magnet from a subwoofer cause trouble for an amp? I doubt it. Reasons why. Look at a bookshelf stereo. Check and see how close the speakers are to the receiver. I don't see them having problems. Look at guitar amps, aren't the amp and speaker built as one unit? The ones I have seen is yes and they don't have problems.
This is my opinion and I gave reasons for it. Plus, it'* better than no answer.