People are right about polarity. Make sure you get it right. (I used to work as a recording engineer) Hook one up backwards and you will defeat the whole purpose of speaker upgrades! And the really annoying thing is that it will not be totally obvious, you will just think that it's not sounding as good as you thought and not sure why... Basically you will have different speakers fighting with each other, one will be going in as the other is going out, when both are supposed to be going in and it just messes with the sound.
(unless, of course you hook up every speaker backwards, then I guess it would be ok!)