your speakers can get some interferance from the metal that the magnets are up against, since that metal will warp the magnetic field slightly, I'm using this principle to change the sound of my tweeters, and it actually helps in my application. that said, it's not a huge difference.

If you add a sub, it'll probly wreck your speakers, which is a good place to start a good SQ system... ditch the rear speakers and get a really nice set of componants, like the DLS iridiums, you'll never miss rear speakers.