This is the stupidest argument I have ever heard.
Yes, speakers have magnets, but the magnet has to be strong enough to pull on the little stylus thinngy inside of the Hard Drive. If your subwoofer makes every pen, paperclip, and metal object fly to it, then I would be concerned. However, the magnet is not that strong so its not strong enough to wreck the hard drive.
I put a paperclip up to my JBL creature's subwoofer and it didn't pull at all. The satellite speakers attracted the paperclip when it was 1cm away.
Experiment:
1. take a paperclip and put it up to the speaker of your laptop
2. take a paperclip and put it near the latch of your apple laptop
Both the speakers and latch IN the computer have magnets. The latch magnet is quite strong.
Unless you are an idiot, you will realize that the speakers that go on your desk also have magnets in them too. They don't attract every pen and paperclip in a 10ft radius and I can say quite confidently that they do not kill your HD.
Calm down, its not going to wreck your computer.